Your salary will have to do with the position you hold and duties you perform. If you do 100% research in academics with either degree, you'll likely earn pretty much the same. The best money to be made at this time is in private practice in certain specialties. The MD/PhDs in my department are paid more than the PhDs, but this is only because they have a clinical component to what they do and hence are salaried somewhat as MDs. I still don't think they're being paid as much as the 100% clinical guys, but I'm not sure about that. In other less lucrative departments, I don't think the MD confers you any salary advantage, but it certainly does help you get a job.
If you go into industry you may find niche or upper-management positions as a MD/PhD that will increase your salary over a PhD. But this again is a position thing.