Our users love all these types of connections and we haven't heard of issues with BT, Wi-Fi (via Ad-hoc) nor with our favorite solution: musicIO for MIDI over cable. We would know about issues with any of these because we are in touch with users who are using ALL of these apps.
That said, we're hearing about Wi-Fi problems on Windows. Ad-hoc connections on Mac are not that easy to set up and they don't persist, so you have to really get control of your Mac's power situation. If the Mac ever powers down or sleeps, you will have to reestablish.
Anyway, my own experience is that Wi-Fi is rock solid with a Mac, and I personally use Wi-Fi sometimes and musicIO other times.
But anyway, this doesn't really answer the question: I think that both are equally reliable (BT & WiFi over ad-hoc). But test it for battle! Leave the iPad on for 48 hours with the computer on for the same and see if the connection persists. Etc. etc... ultimately these are empirical questions that need to be handled as such.