Once you're inside they're identical, of course. I like the view (Downtown Disney) better from Congress Park, and since I end my day at Pleasure Island I enjoy the walking path from the Marketplace.
The Springs was definitely more convenient to the main pool, fitness center and food - and there is another path to Downtown Disney that enters over by the West Side.
What's best for you depends on what your plans involve. If you want to use the main pool & will frequent the spa/fitness/food areas the Springs is probably better for you. I don't mind being "removed" from that stuff, so Congress Park is better for me.
Hopefully you'll have a great time no matter where you end up. Good luck in the half marathon - I'll be running too!