MDPV is mostly a straight stimulant, somewhat jittery, not all that functional. Fiendy too. Known for "wankathons" - tends to produce significant increases in sex drive.
Mephedrone is an entactogen (somewhat similar to MDMA), not just a stimulant, and as such, none of these straight stimulants can be properly compared to it. Imo, sort of an apples-and-oranges comparison. It's like comparing MDMA to speed, they're different types of drugs.
Mephedrone, as well as Methylone and Butylone are entactogen/stimulants. Most people afaik, have said that mephedrone is more fun, but more fiendy and harder on the body (i'm afraid to touch meph).
MDPV, Ethcat, and Buphedrone are straight stimulants.
Re: bang-for-buck, worth noting that the doses on those three stimulant RC's are about an order of magnitude lower (give or take a factor of two) than for the entactogens. .