The Seiko movement is quartz, meaning it takes a battery and the seconds hands will have a more noticeable "tick" to them. The A7750 is a mechanical movement, an Asian copy of an ETA 7750 Valjoux, which is a common Swiss chronograph movement used in a lot of gen watches - meaning it has no battery and the seconds hands move more smoothly.
Personally if you're new to buying reps or you're new to watches in general, I'd go with the quartz movement. The A7750 is not the most reliable clone movement, and it's fairly expensive to have serviced if you have problems with it. The quartz movement is much cheaper and a good way to "test the waters" with reps - just keep in mind what iggypop said, the factories don't generally put as much effort into quartz replicas, but it looks like a pretty decent copy and I think you'll be happy with it. Quartz chronographs are generally easier to pull off than regular quartz watches because the running seconds hand is smaller and less noticeable.
If you like what you end up getting, I'd try some simpler mechanical movement watches after. Of course there's no reason you can't go with the 7750 either, any watch movement can die prematurely, it's just something you should always be prepared for when dealing with reps (or gens, for that matter). If you decide to go with that one instead, you should take a look in this section:
http://forum.replica-watch.info/vb/forumdisplay.php?f=250 and read the stickies, particularly the
7750 User Guide.
Good luck with whichever you decide on
