The infinity sub you've mentioned is a fine sub, but the perfect series is probably a lot better and more comparable to the w6 series. Although the w6s came out about 7 years ago, if I'm remembering correctly, these were the subs that single handedly changed the engineering and goals and design of car audio subs forever. It was unheard of the very tiny boxes they required compared to all the rest. I can't believe that they are still a very strong competitor as far as subs go.
If you were having trouble over the making a decision between an infinity perfect series sub and the w6, I'd say go perfect series, but the w6s are very tight, clean, strong, hard hitting subs. can't go wrong
only problem is that you basically need 3 of them to make a proper load on your amp, unless you got an amp that likes either a 6ohm load or a 3 ohm stereo load.