50 Cent had a higher peak. He was selling millions of records and pretty much everything he dropped was a hit. This was '03-'05. By 2007, he started struggling and losing that sales war with Kanye was a catalyst in his downfall. By 2008, it was flopped single after flopped single and he's competing with Rick Ross. We're not talking Rick Ross in 2010, we're talking about a Rick Ross who at that point wasn't dominant and was barely going Gold. Since, 50's struggled to get a release date.
T.I. continued to get bigger and bigger from '03-'06. He actually did re-introduce a sound to the mainstream. Trap music existed sonically and content wise before T.I., but it didn't have a name or real definition. It was just a reference before T.I. T.I. wasn't moving the type of units that 50 Cent was at his peak, but he was dropping hits. Just when people thought T.I. hit his commercial ceiling with King in '06, he extended his run and peaked in '08 with Paper Trail. That was his biggest album and it had his biggest hits. T.I. was still going Gold in 2012 and had hits as late as last year. The same can't be said for 50.