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Neither.
You'd be better off saving a bit more and getting a 2nd hand Takhashi FSQ106N or if the budget will stretch a 2nd hand FSQ106ED.
You'd never outgrow those and you would not take a loss when you do the inevitable upgrade with the other 2.
Astrophotography is very demanding of the optics and neither of those you mention will meet high end needs.
F9 is too slow. Doublets are not APO but semi APO and have chromatic aberration fluorite or otherwise. F9 is an attempt to hide that CA. But it may show in images as magenta or blue haloed stars. It would take a lot longer to get an image and F9 is pushing 102mm of aperture too much. The chromatic and spherical aberration will reduce sharpness.
F7.5 is a bit slow but OK. Others can comment but I imagine the focuser will be inadequate for astrophotography. 127mm is good aperture though but you aren't going to get any galaxies except for the few very brightest.
FSQ's are F5 and quadruplets.
The 106N has 2 fluorite lenses and the ED model has ED instead.
106N has better colour rendition than 106ED and a stronger focuser which does not flex. It is the better bang for buck. But it does have slight vignetting on bright stars near the perimeter (they have a dark bar going through like a worm hole!). A minor flaw. No scope is perfect.
106ED colour rendition can be improved by flocking the interior of the scope as it gets a green bias from inside the tube from something (the black paint or the lens coatings?
F5 and 106mm seems a sweet spot for 4 inch refractors in my opinion. Every 2nd top image you see is from FSQ106ED. FSQ106ED though has been plagued at times with focuser flex with heavy cameras. There are several models to try to cope with this. I am not sure they ever have fully. I had one that had virtually no flex. I was lucky it seems from the number of posts about this topic. 106N has a stronger simpler focuser with a proper focus lock that works.
Basically 4 inch scopes work best as widefield imaging machines. 4 inches is not a lot of light gathering power so they work best in this zone. Super high quality lenses in the 4 inch range could be pushed a bit more but you are now pushing things a bit and the job is done better by larger aperture - either a refractor (very expensive) or a mirrored scope of varous designs.
Greg.
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