The old knock on Bell - which may apply to others - is that he scores big when someone else also does. He isn't the guy to hold an innings together but a guy who tons up in easy conditions when someone else is also scoring heavily.
EDIT - a quick look shows that Bell was the only centurian in England's innings on 4 of 17 occasions whereas KP was the only man to score a hundred in the inning almost 3 times (I lost count, it is about 11 or 12 times) that number in 22 innings.
You know, others to score a hundred is not really something you can control as a batsman. And, if Bell has filled his shoes when others did, how come KP has not in those innings? I don't think he decided that 'Ow this track is easy, let's get out, somebody else will score here'. Eventually, as we have seen, their stats are pretty similar.
Anyway that's just me, I never really believed in these things such as clutchness*. I put it this way, if cricket was an open market game (which, thank God, is not, let me be clear about this) and I was the GM of some franchise, I'd be extremely happy to get Bell at a much lower price (60%? You know, at the Top End increases are exponential) than KP, extremely good player, but one which I believe would have an exaggerately inflated price due to reputation. Then, if others disagree with me... fine. I got mine, you got yours.
*which really helps me as a player also. I go and play my best game, whatever the situation. If the bowler beats me, good for him, he was better. But pressure is for ignorant wimps.