I have a 13" MBP and an iPad 2, plus a desktop at home.
To me, there isn't much more I would get out of the 15" MBP besides the larger screen, so I saved $ and went 13".
The MBP and iPad are both portable, but are quite different. It's kind of like how the first non-console TV's were called portable. Sure, you can travel with either device, but you won't.
I only take my laptop if I know I will definitely need it--yes, it does some things better than an iPad, no doubt about it. But traveling with a laptop isn't just 'grab and go'. You'll have a whole bag with your charger and other attachments as well. People simply dont carry all that crap everywhere and it's hard to hide in a car, which is inviting for thieves.
With the iPad you can just pick it up and leave. The battery life is amazing and it fits in most vehicle glove compartments.
Laptops are portable, but iPads are truly mobile devices. If you can do a 13" MBP and an iPad 2, you're making a good choice.