There are direct, fast (ES) trains from nearby stations in both cases, however Venice has the edge:
Buses from Marco Polo airport stop outside Mestre station. You can research Munich airport connections on the airport's website.
The nearest significant airport is Verona. It has far fewer flights than Munich or Venice, and doesn't have transatlantic flights, but is still possible from Minneapolis-St Paul with only one transfer.
Verona - 1 hr 45 mins and has regular trains
Venice - Fast train is 2 hours but not many in a day. Slow train takes longer.
Innsbruck - 2 hours with international train and is every 2 hours.
PS - If you are interested in Bolzano then have a read of my site. Bit basic at the moment though.
Doh - can't even post my link properly :(
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