Mark, as you've noticed they have different attributes so it really comes down to what you value the most.
The 100 is almost a f/stop faster than the 120, but the 120 should resolve more detail and collect more faint light due to its larger aperture. A fast 4" scope probably wouldn't be my weapon of choice for small, faint objects, but some test images I have taken have pleasantly surprised me. At 840mm focal length vs 550mm the 120 is always going to win in this respect. Throw a small, sensitive sensor in the mix and it should be interesting, but a longer focal length instrument still will likely do better.
There's also size and weight to consider, so it depends on what mount you're using. The 100 surprised me when I got it. Didn't expect a fast 4" scope to be the size it is. The dew shield is huge.
There aren't all that many targets as large as Andromeda, Orion nebula, Eta Carina nebula...another solution would be to throw a larger (full frame) chip at the 120 which would give it a larger FOV than the 100 with APS-C sized sensor like an 8300. I recently attempted Andromeda and shy of some faint extension, it just about fits, see here
http://www.iceinspace.com.au/forum/s...d.php?t=138095
My feeling is that the sensor you choose and how dark your imaging sky is will have just as much effect on how quickly you collect good data.