I think England should choos Anderson over Collingwood for as much psychological reasons as anything. Selecting Anderson shows the same attitude, picking a side of four quicks to really trouble the opposition and take the attack to them. Collingwood would be the 'saftey first' option, but a) that can backfire as seen plenty of times in the past, the extra bat does not automatically guarentee a good total, and b) it sends a negative message both to the Aussies that we aren't as confident as we should be having outplayed them 3 tests in a row and also to our own players that the selectors and coach don't think we can win without having a 'just in case' guy. Of course, the message would be inadvertent, but that is irrelevant. 2-1 up and playing the better cricket, we should be positive, look to win at the Oval and bury the series once and for all.
If the Aussies win the toss, bat and get off to a good start, say 240-3 at tea, I'd rather have an extra quick to help the attack than a guy who bowls dibby dobber medium pace (but don't worry, he can bat a bit too!). I think Anderson has a lot of talent, anyone who saw him down under in 2003 and at the World Cup couldn't deny that. However, as he has shown since then he is also very raw. He has had a solid year of county cricket (finally) and has done decently, if not exceptionaly, but I have a confidence that sooner or later he will blossom into a true Test-class bowler, he has all the talent needed. Well, either that or he's a bowling Ramprakash.....
Bottom line, Collingwood wouldn't make up the runs with his batting (and fielding) that we'd lose by having only a four prong attack.