Robin's answer was best for your question but I'd like to throw in a caution, which may be totally unneeded but just might be helpful: pay attention to how any discussion about money is going.
If one individual is problematic and really really wants to make sure everything is going to be fair someday, when there may or may not be any money, look out: you may find yourself having to justify your 5 cups of coffee to his 3 at a later date.
Okay, that said, if everyone is groovy and just no idea how to tackle a split I would vote for equal shares and expect equal effort. The company won't succeed without a backend, and equally won't succeed if no ones sells it. If the marketer has nothing to sell early on they can be building social networks to sell into later, building buzz, teaser pages, generating test data for the backend, something. If the backend has less to do later they can be optimizing, planning for expansion, making calls for the marketer, again, something. IMHO experience there's no shortage of tasks to do, just (sometimes) shortage of commitment.