I am a co-founder of a nascent third party payments aggregator (TPPA) called PayGuard (currently in stealth mode - Beta version is being built), and our system inherently includes automatic fraud prevention; so we not only save money by eliminating the screening process, but we also save by virtually eliminating all 'dispute resolution'-leading scenarios.
As far as a comparison for Stripe, they seem to be similar to Braintree, but geared more towards early stage online merchants. The following link may answer some of your other questions:
http://www.quora.com/What-are-the-advantages-of-using-Stripe...