I was just wondering what you finally settled on??
I started my adventure in astronomy with 15x70mm binos, which I still use on an Orion Paragon-Plus mount. My current lineup includes 7x50mm, 8x40mm, 10x50mm, 15x70mm, 20x60mm, 20x80mm, and 25x100mm; although I've only listed the 8x, 15x, & 25x in my signature because they're my primary astronomy binos.
My 25x100mm probably give the best "wow" views, especially to those new to the hobby (such as myself). But they're a beast of a bino at ~10lbs. Although my 15x70mm are actually pretty crappy quality (standard Celestron SkyMasters) I use them quite a bit because they work perfectly on my p-mount. The 8x40mm I got are quite heavy and will probably be upgraded at some point (along with the 15x70mm), but they are super handy for searching the sky.
As far as difference in view, the 100mm definitely bring in more light but the 70mm do great on DSOs too. The 40mm with show a faint smudge of a DSO but that's about it. They're definitely better on stellar objects than faint objects. They'll show the Galilean moons but just barely and Saturn looks mostly like a disk. 15x70mm show the moon's great but Jupiter is just a washed out disk and Saturn is a elongated football shape. The 25x100mm with actually show the equatorial bands (best at dusk when contrast is higher) and are able to separate the ring from Saturn.
To your original post: IMHO, 15x70mm are probably the best to use along with your 8" because they're able to show a lot without being too much of a hassle to setup. Although the 25x100mm are freakin' amazing, transport & setup is definitely more involved especially with a p-mount big enough to handle them. A heavy duty tripod that'll handle say 25lbs will work (that's what I use), but you'll want to limit your targets to below 60-70° elevation. The 15x70mm can be used on a lot lighter tripod/p-mount.
So, IMHO, if you're wanting big binos to use with your 8" Dob like I do, then 15x70mm. If you'll be using them a lot by their self, as in setting up the 8" or the binos, then the 25x100mm work awesome. Either way a smaller pair that you can use handheld is also extremely handy to have.
- AstroApe (aka Matthew)