There has been a few reports from Orange mobile that they will support UMA on the Z10 but that was a few weeks ago that they said it. Other then that, no one has heard anything.
It could be that the Z10 does support it, but the carrier needs to add it in the software / firmware. Blackberry reps never have answered any of these questions at all yet. Normally when the phone does not support something, they tend to say it.
Pauly and Meowmix, I don't believe the OP was asking about UMA calling. He/she was asking how the phone will connect if multiple connection options are available.
Toronto_User, your phone will default to the Wi-Fi connection first if there is a strong Wi-Fi signal available. You don't have to adjust any settings for this, your phone will connect to Wi-Fi automatically. However, make sure that your Wi-Fi signal is strong enough and that it doesn't get dropped. If your phone connects to Wi-Fi but then loses the connection, it will revert to your network signal with no indication that it has done so.