Steam definitely has xbox beat on visibility of new releases. The xbox live service focus's on xbox live arcade titles and retail releases heavily and clouds everything else behind that.
XBLI was awful from the start. Impossible to judge quality of a title, and often difficult to even see what a title was about. There doesn't seem to be an effective way to sort titles or display information. Computers do this far, far better, which is why I'm happy you're able to buy cellphone games via computer. That said, Steam isn't too much better on this front, but at least games on Steam tend to have an internet presence elsewhere to get information.
XBLA has gone like XBLI by this point. It's been about a year since I've purchased a game there, or much even browsed it. Amazing at first, but by now the selection is too great, the quality too varied and the ability to tell what you're looking for too low for me to bother. I'll spend more time looking for a game than playing a game.
At this point calling both those sections failures seems right.