The Affinity has Gotoh style tuners with screw-in metal sleeves; the Bullet has much cheaper but usable tuners (shaped like mid-late 70's Strat tuners), with "chome" coated plastic "press-in" sleeves.
The Affinity is available with all maple or maple/rosewwood necks; the Bullet is only available with maple/rosewood necks.
The trems and saddles look the same to me. The pickups look the same too, although I've gotta say that the neck pickup on the Bullet is one of the nicest sounding neck pickups I've heard... very vintagey sounding.
The trem cover on the Affinity is the same size and shape as the MIM Fender Standard Strat; the trem cover on the Bullet is fatter and shorter, and has a different screw pattern.
Affinity is the only Squier with the narrow neck AFAIK, but bear in mind it's a fat narrow neck. Affinity as an Alder body, Bullet has a Basswood (or plywood in older models) body. The Bullet is the less expensive model.
If all you want to do is play the guitar, not mod it, then the Affinity might be the way to go.
But if you're planning to mod the guitar anyway, the Bullet is a great modding platform, as the neck is really nice (it has a vintage size and shape headstock). The basswood body is nice too, and the body cavity is shielded with conductive paint. The Bullet is available in cooler, vintagey colors too (daphne blue and fiesta red); the Affinity isn't available in the vintage colors.
I went with the Bullet and modded it :
http://www.tdpri.com/forum/1152435-post101.html
