It depends what you're trying to process. Keep in mind Dwolla is it's own network. It doesn't process VISA/MAST/ other like Stripe. In theory Dwolla is another payment form that could easily be built into Stripe but the two do very different things.
Using both obviously gives you the ability to do cc/debit transactions for those who want and have the upside of when someone pays using Dwolla.
Using both obviously gives you the ability to do cc/debit transactions for those who want and have the upside of when someone pays using Dwolla.