Help picking the right sub

350w will be more power to the sub but sound quality has nothing to do with power ratings...

you can have a dual voice coil wired in 2 ways.

parallel and series

a dvc 4ohm sub can be wired parallel to have a load of 2ohms on the amp
a dvc 4ohm sub wired series will have a load of 8ohms.

it depends on how you wire it
http://www.edesignaudio.com/wiring.php

for example my previous subs were 2 dvc 4ohm subs
but since i wired both subs in parallel it had a load of 1ohms

of course you need an amp that is 1 ohm stable

so check your amp if its capable of going to 1ohms, if it can you could get around 600w

If the subwoofer you are looking at comes in both 4 ohm and 2 ohm versions, it is likely that they are a Single Voice Coil (SVC) design. The previous post refers to Dual Voice Coil subs. The way to tell the difference is by the number of speaker terminals- the SVC will have two- one positive and one negative, and DVC will have four- two of each. If it is a SVC and you plan on running a single sub, it will only connect one way.
It is a special amp that can safely run at 1 ohm mono, and unless you know for certain that it can handle the load, don't hook up your amp this way as it will likely kill it. The lower the impedence (as measured in ohms) the more stress it puts on an amp.
The higher the impedence, the more control you can have of your speakers, and the better the potential sound quality. This is why home audio equipment operates at an 8 ohm impedence standard (and an increasing number of competetion sound quality vehicles) instead of 4 ohms and lower in vehicles. Most feel it isn't an issue with subwoofers in vehicles, as they are only reproducing lower frequencies which aren't expected to be as detailed as the upper frequency ranges. Lower impedence speakers are a good way to pull a lot more power from an amp, but as with everything, there can be tradeoffs.