3G vs WiFi - which does the VZW TP2 use when both are available?

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I haven't seen this question addressed anywhere, and I'm just wondering if there is a defined strategy that the Verizon TP2 uses when it has both a 3G and a WiFi connection available.

The second question is whether the user can alter that strategy if it's not what they prefer.

I've seen some funky erratic behavior at times: e.g. it won't find or use a 3G connection after I leave an area where I was using WiFI, until I turn off WiFi, or other times it doesn't seem to see a WiFi connection that's available until I turn off the 3G Radio. These are not consistent behaviors, which is why I ask the question.

Thanks!