Your choices are good ones. If portability is really important, the 120 is wonderful. It's light weight (8 lbs... IIRC, mine is 11 lbs with rings, finder, NV, and a handle), small enough to handle easily, is carried nicely on almost any mount, is very fast to set up (no collimation) for quick looks and the price is right either new or used. If you are using a filter (either long pass IR or H-a bandpass) you will never notice star bloat. Although I now know I was not getting full field illumination on the NV sensor with mine, it never really bothered me, especially when used visually. Where I saw it most prominently was in photos. I used an Orion .8x reducer made for refractors, which took it to f:4 at 480mm. The 480-600mm FL was very useful for many medium sized nebula. I also used it occasionally with a 2x barlow when taking images of small globs, galaxies & planetary nebulae. I took a lot of phone photos with the ST 120 and was happy with most of them. For visual use, I was completely satisfied.
Advantages of the ST 120 over a 6" Newt are pretty much limited to the convenience factor (size & weight), and under some circumstances, it may have a sharper image of stars that is easier to maintain. The 6" Newt I had was quite excellent. It was barely heavier than the ST 120 and was easily mounted and controlled by both my manual alt/az and my AZ Pro goto mount. It had the extra step of checking collimation, but it held collimation quite well, needing adjustment only occasionally. The scope is native f:4 with a FL of 600mm which is the same as the ST 120. The big advantage is using it at f:2.8 as a prime objective with the addition of the ASA reducer. The drawback is the cost of the reducer from Teleskop Service in Germany. There is a substantial difference between f:4 and f:2.8 when using a scope visually with filters. A benefit to you might be double duty of the reducer, as it is probably quite useable in your f:4.7 Dob.
Keep in mind that my use of the ST 120 was pretty much limited to prime focus and I found it very good for my purposes, both visually and for imaging. I tried it in afocal with a 2" 55 Plossl and a 1.25" 40 Plossl, but was never really satisfied with the image, especially for photos. This is a personal preference based on my tolerance of outer field aberrations. But the advantage is that very fast focal ratios can be achieved with the right afocal eyepiece, even more when used with a supplemental reducer. If your focus is on the center FoV, you many not even notice field curvature or astigmatism toward the edge of field.
A fast objective focal ratio is a friend to NV. The faster the focal ratio is, the more compelling your NVD image will be. The faster the focal ratio, the better your H-a filters will seem to perform, as the image will be brighter and will reveal more contrast.
I don't think you will be disappointed by either scope. Since you already have an f:4.7 Dob, and convenience seems to be a high priority, the ST 120 seems to be the likely candidate. Good luck.