I think this is a very legitimate comparison. Consider that a trade will need to be made in order to acquire Boldin. Bryant can be signed off the street but comes with documented baggage.
If you had to pick one of them would you take Boldin who seems like a better locker room guy as well as a better talent but will likely cost at least a 3rd and player(s) or Bryant.
My gut says unless Bryant comes at a discount, I'm going Boldin all the way.