On the Argos website the Sony W320 is £129.99 and and the W350 is £149.99. Both of those are actually very good prices as the cameras aren't even available for sale on a lot of other websites, they're pre-order only.
There isn't a huge amount to choose between these 3 cameras. Matt is right about pixels, it's not simply a case of "more is better". In reality though the Sony S350 is the best all round camera of the 3 you listed, not by much but it is the best.
However, there are literally hundreds of compact cameras on the market and, for a very reasonable budget of £150, you can do better. If you want to consider other options (if not then just stop reading here) then have a look at these cameras:
The
Panasonic DMC-FS15, it's around £135 and has a better zoom than the Sony's or the Nikon.
and...
The
Ricoh R10 is a tiny fraction over budget at £153 (ish) but it's the one I would buy without a doubt. It's got a 7.1x zoom (28-200mm equiv for those who are interested) and a 3" HVGA screen (the other cameras mentioned here have smaller, lower resolution screens).