Native BB30 crankset or adapted Shimano?
If the frame is made for BB30 I'd recommend using that. 

Advantages of using native BB:


BB30 spindle is thicker (30mm) and stronger than a gxp (24mm)
No extraneous parts


Advantages of using adapter:


Ease of service - no bearing press needed except for initial install, after which you will have a threaded BB that's easy to remove.  BB30 requires relatively expensive tools to service, e.g. Park Tool bearing press and puller, which most enthusiasts don't own.  Also, the fact that you only need to service a BB30 once in a while (maybe once or twice a season) makes buying the tools less attractive when one can just take it to the LBS and have them pull your BB30 for relatively cheap.
Q-factor - can be an advantage or disadvantage. Your stance will likely be wider with the adapter. I consider this less desirable but it's all about the fit. I like a narrow stance.  You may have knee pain if your q-factor is too narrow or too wide