First directly answering OP's question: if you've got one tyre wider than the other, run it on the wheel taking the heavier load: rear. The tyre can/should be run at a lower pressure and will (counter-intuitively) roll better and be more comfortable.
Regarding choice of tyres, I recommend looking at this leading source of advice/fact.
This link is to a comparison of three tyres, but there is much more.
Also there have been a few recent threads on tyre choice -
this is one such.
Don't get anything made by Continental if you need any more puncture resistance than a stripe of toothpaste. They do have very good grip and rolling characteristics, but if you ride more than a couple of miles on winter roads, you'd be better off riding on the bare inner tube, as far as puncture prevention goes.
@nickyboy gives good advice (ref there being many PoVs) and just to reinforce that: IME and IMO (and like the others who responded immediately above) Rr's advice above is complete rubbish (see the link above for facts) - don't let him put you off Contis, though the Gatorskins are heavier, less puncture resistant and roll slower than GP4Ss. My (two sets of) Conti GP Seasons in the last 16 months have done 12000km and suffered one puncture. I shall be fitting some Michelin Pro4 Service Course next (as they were cheaper for a pair, almost as good puncture protection and less rolling resistance). These are currently about £23 at Merlin Cycles, and the 25 comes up a millimetre wider (ie 26mm) apparently (which btw is the same as the GP4s 28s - 26mm).
