2 Ohm vs. 4 Ohm?

So i'm shopping for two 12" subwoofers for my truck but i'm not sure if I should buy 2 ohm or 4 ohm subwoofers. I got a 2 channel amplifier from a friend for now but I plan on upgrading to a better amp later on. Here is the amp that I currently have:

So i'm shopping for two 12" subwoofers for my truck but i'm not sure if I should buy 2 ohm or 4 ohm subwoofers. I got a 2 channel amplifier from a friend for now but I plan on upgrading to a better amp later on. Here is the amp that I currently have:
http://www.sonicelectronix.com/item_67954_Crunch-DRA850.2.html
Thank you for your help i'm still new at this

Other answer:

Andrew Perez:
In general, you want your speakers to be as high an impedance as available. A nominal 2-ohm *impedance* speaker may drop in actual *resistance* to well under an ohm. The difference? Briefly, and with lots left out:

Impedance = AC resistance. A speaker is a linear AC motor and behaves differently at different frequencies, just as AC motors behave differently at different speeds or if fed from a Variable Frequency Drive – which is, effectively, what your amplifier is. Again note Lots Left Out.

Resistance = DC resistance. What you would get by putting an ohmmeter on your speaker. It measures only the resistance of the voice-coil to a small DC current. It is pretty much constant. So, you cannot get a valid reading of a speakers impedance from an ohmmeter.

Now, many solid-state amplifiers are able to drive very-nearly-0 ohm loads, but it is still not good for them. Most amplifiers are designed to delivery their largest (safe) output at into about a 4-ohm (nominal) impedance. That gives them sufficient range to cover the entire audio spectrum without dropping too low.

N2Audio:
To put it simply — get a pair of subs with dual 4 ohm voice coils.

That will match up well (temporarily) with your Crunch amp as you'll be able to bridge the amp and wire the subs for 4 ohms which will get the most power from it — helpful since it's not very powerful for a pair of 12's.

When you are ready to upgrade you can rewire the two d4's for 1 ohm which is the ideal impedance for many quality ~1000w subwoofer amps.

About any type of sub could work, but none are as logical as the d4 option IMO.

eric:
this is not something to trust to some idiot on here go talk to someone at the store where you bought it or some other audio store they love to talk about this and will help you otherwise you might get stuck with something you cant return. if you have 4 ohm at 120 watts two ohm can push 180 watts depending on your amp. ive made the mistake of buying **** i cant return and possibly destroying your speakers dont listen to anyone on her talk to an expert
Ordinary Guy:
4 ohm do 2 ohm even exist
Dixon:
If you are using an active subwoofer speaker the impedance is irrelevant. If it is passive and you are using a crossover so that it is the only speaker in its frequency range, then it should ideally be matched to the amplifier, which is likely to be 4, 8 or 16 ohms, depending on its power output.