The NX actualy has some mods out there that will give it proven performance increases. The MX3 is mainly a bunch of rice stuff like body kits and such. You are starting off with a better engine with the NX compared to the MX3. The Mazda isnt really a sports car, its more of a commuter. The NX will run circles around it in a straight line or in the "twisties". The v6 MX3 has a whoping 140hp i think(correct me if Im wrong) the NX has 140 as well, but in a 4 bannger. So think about it, which is more efficeient. NX problay would get better gas mileage as well. Then the question comes down to which quirkyness you like better. The frog look to the NX(which I love and want one bad), or the weird hatch design on the MX3. I would rather put money on a NX. 3000, thats a good price just depends on mileage and other common problems.
NX the rear hatch supports like to go dead, so if the hatch doesnt stay up, thats common but sorta expensive to fix.
Motor mounts always in question on a sr20, but only 25 bucks a pair to fix for ES inserts.
5th gear pop out. In 5th crusing if you give it gas the gear falls out, not a big problem but does need the tranny almost rebuilt, couple hundred dollars, but common.
T tops leaking, very very common, but somewhat fixable with various methods.
And a few others that some may list. But I would say the NX is a better buy as long as its in decent condition