I've owned 2 SX basses and was happy with both of them. Basically, I wouldn't bother with a Squier affinity, because an SX is pretty much the same thing (cheap Asian-made Fender copy) for less money. With Squier I like the Vintage Modified and Classic Vibe lines, but they're a bit over your budget unless you buy used (they run $300-400 new).
Of the two basses you originally posted, the SX and the Peavey, you have very different animals, especially in the neck. The Peavey neck is very slim, like a Fender jazz. The SX neck is wider and thicker. That's really just a matter of preference. The SX jazz copy (the Ursa 2) has a more jazz-like neck, of course, but even that isn't as slim as Peavey necks. Personally, I like a slim neck and would probably take the Peavey over an Ursa 3, but either is a decent bass.
Yamaha is also a good solid brand, you might look at the RBX basses from them.