Yes, I know this comparison has been posted before by other people, and in much more detail and much more eloquently expressed than I'm doing so here. But I don't recall seeing any similar posts for several years, so I figured I'd revisit this topic by posting my recent experiences.
A little background first. My Takahashi FS-102 (a 102 mm, f/8 doublet apochromatic refractor with an outer calcium fluoride element, inner mating element of unknown glass, and 820 mm focal length, manufactured in 2001) was purchased from a fellow CN'er a little over 5 years ago, in summer 2014. At that time Takahashi wasn't even on my radar, and I already had a nice Stellarvue SV102ABV apo doublet with a LOMO OK-4 ED element and a lanthanum mating element. But the Tak was listed at a nice price by a friend with whom I'd done multiple previous gear transactions, and so I jumped at the chance to buy it. I figured I'd compare it to my prized Stellarvue, have some fun with it for a few months, and then resell it. Well, suffice it to say that as nice as the SV scope was, it didn't take long before it was on the market, having been outclassed by the Tak. The Tak was really that good!
Fast forward 5 years later, to the summer of 2019...
I wasn't really looking for another 4" refractor, especially since I was still enjoying my Tak FS-102, and frankly I had been spending much more time lately observing with a larger TEC 140. Although the 4" Tak can get by with a smaller mount, I typically prefer using it on my robust DiscMounts DM6 and Berlebach Planet mount/tripod combo. And if I'm already going to lug out that heavy mount and tripod, it's not really that much more effort to take out the bigger scope, the TEC 140 instead of the FS-102. But as I was browsing AM one day this summer, an ad for a nice condition late model Astro-Physics Traveler (a 105mm f/5.8 apochromatic oil-spaced triplet with FPL-53 glass and 610mm focal length) showed up. Its serial number and black pebble-finish indicated that it was produced during the final run of these scopes, in either late 2003 or early 2004. And even though I hadn't been looking for another scope at that point in time, I had always wanted to try out this legendary scope, and I figured if I didn't jump now, the scope would likely be sold within hours. Fortunately, I was the first to respond, and about 2 weeks later the scope arrived in excellent shape. It's much more compact than any 4" refractor that I've ever seen, and in fact my first reaction when I was unboxing it and saw the case for the first time was "no way could that case actually hold a 4" scope!" In fact, the case is the same size (or maybe a little smaller) than the cases that I keep my FS-60Q and SV80S in.
I've now had several opportunities to test out the AP Traveler, and I'm really thrilled with my purchase. But what I really wanted to find out was how it compared to my prized Tak FS-102. Finally, last weekend I had the right combination of good weather and lack of other competing obligations, and I was able to spend a few hours comparing these two scopes side by side. Although I've never weighed the two scopes, I believe the FS-102 may actually be a little lighter, as it's a doublet with slightly less aperture. But because it's quite a bit longer, IMO it really benefits from a beefier mount, as I'm someone who really likes my tripod and mount to be rock stable and get out of the way of my observing. I set the FS-102 on the DM6 and Berlebach Planet, but the shorter AP Traveler turned out to be a perfect match for my lighter UA Unistar Deluxe mount and UA wood surveyor's tripod.
This mini-review is already getting long enough, so I'll cut it short, as I don't want to bore anyone by discussing a lengthy list of targets that I viewed that night. I'd rather just post my general impressions here, so suffice it to say that I looked at all the usual summer Messiers plus Jupiter and Saturn and a few double stars. All in all, I had a great time outdoors, spending a few hours comparing two really great 4" apos, in one corner the Japanese FS-102, and in the other corner the All-American AP Traveler. Although the focal lengths of the two scopes are different, at 820mm vs. 610mm respectively, I was able to get similar magnifications by using different FL Ethoses. And honestly, the views were very very close. Had I not had the scopes side by side, I probably wouldn't have noticed any differences at all. And even so, I'm still not convinced that any perceived differences may not have been merely due to momentary changes in seeing. My wife thought some objects looked better in the Tak and some better in the AP. My son told me he thought the AP had better images. But the reality is that when you're dealing with two high end apos, any visual differences (real or imagined) are really just splitting hairs.
What I like better about the AP is its compactness (due to its being f/5.8 vs. the f/8 Tak, and its sliding dew shield), and its superior focuser. In my opinion, the AP 2.7" R&P focuser with FeatherTouch micro-pinion (FT MPA) dual-speed knobs is just as robust and smooth as a full FeatherTouch 3" R&P such as the FTF3035. The Tak focuser is pretty smooth, but has a very slight amount of image shift that I haven't been able to fully tune out, and may be related to the way I've got the FT MPA set rather than due to any issues with the stock focuser. I had never noticed that before I added the FT MPA on to the stock focuser. I do have a FTF3035 which I've used before on the Tak, but I really like the way the stock focuser plus FT MPA looks, rather than the all-black FT, which looks good, but IMO not as good as the classic Tak look.
So the bottom line is that these are both two very nice 4" apo refractors. For astrophotography, I don't doubt that the Traveler would likely be the better choice, but I'm a purely visual observer. For my purposes the Traveler is likely overkill if one only considers the views that it gives, as the views are really very very close compared to the FS-102. But given the extreme compactness of the Traveler, I'm pretty sure I'll be much more likely to take this one outside on a cold winter night, or on a night when I have to get up early for work the next morning and don't have much time to observe and lug a bunch of gear in and out of the house. For those of you who already have a fine Tak doublet apo and are wondering what you're missing by not having an AP triplet apo, the answer really is not much. While I won't go so far as to say that the AP is overpriced (since I place a high value on its mechanical finesse and extreme portability), I can say that the Tak is quite a bargain, especially at today's used market prices. For now I'm glad that I've been lucky enough to be able to own both simultaneously. Maybe one day I'll come to my senses and realize that I have too much cash invested in 4" apos and sell one or the other, but for now I plan to enjoy them both for a while. 
