Basic question but 13 inch or 15 inch???

I was set to buy a 13 inch and then thought about the processor being difficult to change --- I can probably change the hard drive/memory fairly easily. (I wish Apple had charged $100 more on the 13 inch Pro and put a 3.06 in there similar to the one in the iMac if possible, but anyway...). So, $600-$700 buys me a better screen (resolution), bigger screen, better video card/video memory (gaming possible), a decent i5, and slightly bigger hard drive. Plus switchable graphics. I have $600-$700 more in cash, lighter weight, modestly better battery life with the 13 inch. (With a somewhat dated CPU that won't be any newer in 2012.)

This would be my first MAC and I would probably watch video, internet surf, use office, install Win 7, listen to music in some fashion, lightly edit photos, and, perhaps, game. I also suspect another great use for home computers will develop over the next year or two --- who knows what it would be?

Would getting the 13 inch possibly be cheap now and dumb later (or even now)? Tx.