Broxton and Bell are both good examples of show me first.
Closer is such a difficult skill set, simply some guys have it some guys don't. Eckersley went from a middling starter to one of the best closers in the game. It's not all about stuff, it's about mentality, ability to put a loss behind you, and the ability to make the final out.
There have been tons and tons of great setup guys that looked like they would be the next big thing and simply petered out when given the chance to close. Octavio Dotel is a great example. Some guys have been able to make the transition from 8th inning guys to closer.
I'd probably rather take a chance on Bell than Broxton at this point for two reasons. First, he is more experienced, and has had the chance to learn from watching a potential hall of fame closer in Hoffman. And while his fastball is not overwhelming, there are plenty of closers who don't throw high 90's and get people out regularly. The other reason is Broxton is going way too early in the drafts I've seen because tons of people love the fastball. I want my closers to have been battle tested first.
That's just me, of course. Not to say either of these guys will not have fine seasons, but I'm not going to reach for either one.