Honestly both make great motherboards. What I don't like about the GA-EX58 is that is comes with 4 memory slots. This just doesn't make much sense with DDR3. The p6t SE also comes with 3 PCI-E 2 X16 slots while the GA-EX58 only has two but that should only be a concern to you if you actually plan on using it.
However if I may I would recommend the GA-EX58-UD4P its a great board and can be found for $220 @ Ewiz http://www.ewiz.com/detail.php?name=MB-EX58U4P Comes with a $15 mail in rebate. Also Ewiz tends to have promo codes and you may be able to knock off some of the price if you look around. Plus the motherboard supports both CrossfireX and 3 way sli which may come in handy for you.
*edit* if you order it today you can save a extra $20 with this code MEMORIAL20 if not just use the code TAKE10 to knock $10 off.