This ultimately depends on how much space you have available on your computer case (2 slots vs 1 slot) and also your control preference.
The Sentry LX can see all fans at once and displays the time. The screen isn't touch activated and all settings are done by the buttons on the left. The cons are possibly slow input and a rather odd learning curve, as most NZXT stuff tend to not explain that much in the instructions (See here for a review of the Sentry 2 Fan controller, where he points that out: http://youtu.be/OWuvrwZMOO0). The other con is the fact you need 2 slots and its more expensive.
The BitFenix needs only 1 slot and is the cheaper one. It can handle 5 fans like the Sentry LX and uses a touch screen interface to manipulate the settings. The downside is you can't see all the fan speeds at once (You have to cycle though them all to see what is happening much like the Sentry 2 video I linked earlier), and while I can't be 100% certain, it may or may not come with temperature probes (I can't tell from the store page).
Ultimately, this comes down to what you are looking for and what you have room for. If you only have 1 slot, then the BitFenix is your only choice. If you have 2, then if you like seeing all information at once, then the Sentry LX is a clear winner. They should both operate about the same and performance wise, there shouldn't be a difference.