Comparing the 120mm F5 to the 102mm F5, both will give about the same exit pupil / brightness per eyepiece. The CA will be a little worse on the 120mm (see this post by Don
http://www.iceinspace.com.au/forum/s...2&postcount=16 ) The field of view will be more narrow on the 120mm but the limiting magnitude will be fainter.
Alternatively what about a fast Newt? No CA, just coma and collimation to worry about. Those 200mm F4 astrographs are cheap. Maybe on an AZ4?