One of Epic's critically-acclaimed downloadable games was more successful than the other. (Hint: It wasn't the one on XBLA).
2009's Shadow Complex for XBLA was a pretty damn good game, and one of the best-selling XBLA titles of the year. 2010's Infinity Blade for iOS was also a pretty damn good game, and the fastest-selling iOS app ever. Both were developed by Chair as a part of Epic Games, and both were successful hits in their own right.
One, however, was a bit more successful than the other. Speaking with Joystiq, Epic's Mark Rein disclosed that Infinity Blade was "hugely" successful, much more so than Shadow Complex. Rein didn't divulge whether or not he was speaking in terms of download numbers or in terms of profitability, however.
So, our colleagues over at Joystiq did the math: If Rein was talking pure sales, Infinity Blade would have had to sell at least 545,024 copies to beat the number of players on the Xbox Live leaderboards for Shadow Complex. If he was talking revenue - which seems more likely - then Infinity Blade at $5.99 would have had to sell approximately 1,362,557.5 copies to equal the revenue from Shadow Complex's 545,000 sales at $15.
Either way, this just goes to show just how profitable mobile gaming can be in this day and age - if a heavily-marketed and hugely-successful Xbox Live title from a well-known studio can't best a(n admittedly record-setting) iOS game, what incentive do studios have to stay with the former over the latter?