Now first of all, before some Stan gets sand in his vagina, let me say that Eminem is obviously the better artist.
Let me explain. When I'm talking success, I mean by their standards, not a direct comparison.
Slim Shady LP = All the 50 mixtapes (+ Guess whos back) released in 2002.
Their first major releases. Make no mistakes, as big of an album SSLP was for Eminem, all those mixtapes were for 50 Cent, who created the way mixtapes were done, and now everybody is doing it. In terms of content, both are similar too, both are very raw and honest. Critically and commercially very similar.
Marshall Mathers LP = Get Rich or Die Tryin'
Second releases. Easy to see the similarity. Both are their biggest commercially and critically successful albums.
Eminem Show = The Massacre
Third releases. Both hugely successful, and both were a little more radio friendly than previous releases. But artists were much more calm in their approach. Critically, both were well received, but generally not better than their previous albums.
Encore = Curtis
Fourth releases. Both completely disappointing, way too many silly songs and not enough raw honesty and aggression their albums. Inconsistent from beginning to end, and both suffered critically and commercially.
Relapse = Before I Self Destruct
Fifth releases. Eminem's comeback from his hiatus, and 50's comeback into music again after focus on movies. Both were really directed at their core audience, and nothing else really. Although both have cult followings within their hardcore fanbase, they weren't well received elsewhere. They were almost too much back to their roots. It's impossible to say how similar they are in sales though, because 50 Cent's album was leaked 1 month before it's release, unheard of and a killer.
Now, the challenge for 50 Cent is if he can do what Eminem did with Recovery. Find a balance between his core audience, and some great appeal to the wider audience.