I reckon Wenger would favour Szczesny to be honest. Wenger likes youngsters, Szczesny is 21 and played a lot of last season. He's 7 years younger than Foster and arguably already as good. In fact, he's been tipped to go on and become a truly great goalkeeper.
I think Wenger will just keep him and their current number 2. Foster would be an odd signing for him to be honest. And more importantly, it would be Man Utd all over again for Foster, who at 28 no longer wants to stay on the bench - and rightly so.
I'd be more worried about Swansea to be honest. I doubt they're a real threat but as it stands their main target Stockdale is heading for Ipswich.