Up to you, new tranny helps the 05 out of the hole better, plus 7 extra hp.
For me, the deciding factor really was the wait, I opted for a car on the lot that day.
Benefits of 03 - Slightly lower insurance (since it is older car)
More features for less money (since it has depretiated a little bit)
If you're like many and want to mod it, but want to wait til after warranty, you're that much closer.
Benefits of 05 - Quicker transmission.
Extra 7 HP
1st Owner
Longer Warranty period
Later model (i've read on hear that the 04's/05's have less interior rattles than 02-03's credited to working out the kinks.
Either one is great car though!