Hi Mark,
I would pick 100mm f5.5 refractor over 120mm f7 one for astrophotography, especially when starting off with this hobby. Faster f ratio results in a better SNR (for the same apertures and with the same exposures) meaning capturing fainter stuff, so that could compensate for the smaller aperture. Also, shorter focal length would be less demanding in terms of accurate guiding (more arcseconds per pixel).
I would not worry about losing too much detail with smaller aperture. For example, this is a photograph of NGC6334 captured with a 4-meter telescope on a top of a mountain in Chile:
http://apod.nasa.gov/apod/image/0803...w_noao_big.jpg
And the same nebula recorded with a 10-cm telescope on a mass-production mount in Paddy in Brissie:
http://www.astrobin.com/full/201490/C/?real=&mod=
Not nearly the same level of detail of course, but nonetheless the much smaller telescope (1600 times less light being collected per second) does not compare too badly IMO and it allows for fairly satisfying astrophotography.
Having said that, you could always get a reducer for the 120mm refractor, and a 120mm refractor would be a more versatile and potentially more capable telescope of the two.
Just my five cents
Slawomir
Might be worthwhile having a look at this PDF:
http://www.stark-labs.com/craig/reso...s_SNR_RTMC.pdf
EDIT: there is an FSQ85 currently for sale and advertised on this website...