Both have a reputation of being poor under pressure and whilst Bell hasn't shown himself excellent in these situations, I think it's only really true of Clarke.
Other than his debut innings, one innings on the 2005 tour of England and arguably his hundred in India just recently, I fail to recall a time he's scored runs when Australia haven't been completely on top. Obviously it's harder to do so when you're in such a good team, but when you combine that with the fact that his First Class record is pretty modest (granted, he's gone some way to rectifying that whenever he's had the chance lately, but still), the fact that I don't rate him technically against away-moving bowling and the fact that he's seemingly failed whenever Australia have needed runs from him, he still has much to prove for mine. At lot of Clarke's rating at Test level IMO comes from what he's done in ODIs and whilst I don't think you would fall into that trap, a lot do.
Bell also has the reputation of being someone who falters under pressure but I don't really buy into much. A lot of this IMO comes from the fact that often seems to get out playing "soft" shots and, to a lesser extent, the way he looks when he's out there. He's played some really useful knocks for England when they've been under pressure and they're under-appreciated because people remember the soft shot he played on 70 rather than the innings itself. His performances on the tour of Sri Lanka just recently typify my point. He's obviously underachieved and his tendancy to just gift his wicket at times when well set is a serious problem but he's proven more to me than Clarke as well as having a more distinguished First Class record, and I struggle to believe that he could maintain an average over 40 in a middle-of-the-road team having batted all over the place without playing his fair share of important knocks. It's really more a case of me not thinking particuarly highly of Clarke than thinking Bell has achieved heaps, though.