Commenter Nick C says he hears NBC is now narrowing down to a decision between Medium and Chuck, but I hope commenter David is right that it is merely a ploy to negotiate licensing fees down even further.
In the event the information is correct (and though I trust Nick did hear this, it still should be considered rumour) looking at both shows, if it's one or the other I'd probably give a slight advantage to Medium. Medium already has some syndication deal with Lifetime, and it will be at 95 episodes at the end of season. I could see CBS Paramount (producer of Medium) wanting to get to around ~110 episodes to make a better case for syndicated Monday through Friday airings.
On the other hand, Chuck, as commenter Holly notes, is laden with product placement. I'm not sure how much that will really offset costs. Holly also notes that Medium has some scheduling flexibility advantage and it has already shown it can be off the air for many months and not take a huge ratings hit.
On the ratings front, Medium has a slight seasonal lead on Chuck in 18-49 ratings and factoring in DVR viewing, recently even with a weeks worth of DVR viewing, Medium held a slight edge with 18-49 viewing.
In Chuck's favor is it fares much better with 18-34, both on a live basis and with DVR viewing factored in.