Some recommend not making such ratings comparisons and for what it's worth in theory I absolutely agree with that recommendation. Sadly here on planet Earth, particularly on a site focusing on TV ratings, such comparisons are unavoidable.
My biggest issue with the comparison is that there's simply no good apples-to-apples way to look at it. They're on different nights, they're different lengths and had different circumstances. The Voice had a huge sampling with a special post-Super Bowl airing and the next night there was likely still a lot of halo effect. Next week will be more telling as The Voice will be 8 days removed from the Super Bowl rather than merely a day as was the case Monday night.
On the whole, Wednesday's American I dol, which will almost certainly be adjusted up to at least a 6.1 adults 18-49 rating, trailed The Voice's 6.7 adults 18-49 rating on Monday. But Idol was just 8pm-9pm -- and Idol beat The Voice's 5.8 adults 18-49 rating in the 8pm-9pm hour Monday. The Voice obviously had a stronger second hour, but that's always the case (even with Idol when it is two-hours long).
This does raise a question for me of why exactly American Idol hasn't been two-hours on Wednesdays (where surely the second hour would be higher than the first). I can see deliberately scaling it back to an hour if the goal is to launch new dramas or comedies. But all those nights Mobbed was on the schedule at 9pm surely would've been better served (at least ratings-wise) by an extra hour of American Idol. No matter, next week Idol Wednesdays go to two-hours and the comparisons will be a little more apples-to-appleish.