2 ohm have less resistance so you don't need as strong of an amp to push them. I don't think either of them have better anything, it's just that you usually need twice the power to drive 4 ohm speakers.
I have a similar question...I have a 2001 bmw z3 with a harmon kardon system. Unfortunatley it is not very good. I have decided to order an upgrade of the stock subwoofer and amp wich will replace my HK sub and HK subwoofer am to Sunfire/Carver wich will allow me to keep that part stock. The problem is that the main amp which is not big at all something like 15-19 watts rms x 4 needs to stay in the setup. It runs the stock speakers which are at 2 ohms. Are there any good speakers aftermarket that are 2 ohms if not if I do get a good set that are 4 ohms will I lose all the benefits of better speakers because the amp won't be strong enough to push them?
Most experts would recommend sticking with as high a load as possible (4 ohm), because there's a lot less reactive stress on the amp. There are plenty of 1-ohm stable (high quality) amps, but you rarely see competitors running it because they don't want to fry $1500.