It really just depends on your taste. These aren't like two near-identical but differently priced colognes, like say, Creed's GIT or Bond No. 9's Chez Bond (or even GIT vs. Davidoff's Cool Water). These are two very differently priced and completely differently scented colognes. Whereas, like all YSL fragrances, M7 doesn't usually come cheap, Dirty English can be found cheap at many discount department stores or even pharmacies (granted in discount department stores the condition of the product can be extremely rough from being handled, sprayed, and sniffed by dozens of people, and in pharmacies, it's not as cheap.) Not to mention the sheer difference of scent - M7 is a fresh, eager-to-please (IMO) fragrance, Dirty English is a peppery, animalic, "dirty" fragrance - it's out for blood. It's not trying to make anyone happy, and it seems to even trick some people into thinking its fairly good staying power is poor. Really the only thing these two have in common is the wood, and the two types of woods are so different that's not even much in common.
When it all boils down, it's just what you like and what you're willing to pay for.