Wow... I haven't been back to the post in too LONG! We've pretty much decided to rent a car, so it looks like a few of those springs might be in the plans! We're thinking of driving up to the Dolomites and then over to Switzerland. Now to map out those thermals!

Perhaps you realize that most Swiss hot springs should be accessible by public transportation -- train or bus?

Sorry -- meant to add this link to sbb, which can give you all the info you need about public transpiration in Switzerland:
http://www.sbb.ch/home.html

I'm sure they are kja... and I LOVE trains and travelling by them. However, for 4 people with luggage, the convenience and comfort of taking a car in those areas seems to be the better choice.

It ends up being about 35€ / day to rent a compact car that seats 4 in comfort. With lots of driving ( Venice to Switzerland and back ) another 15€ for fuel and insurance is covered by my credit card. 6 days renting a car costs from €300 – €400 all-in.

Train travel may be reasonable in Europe ( except in Switzerland ) but even a non-refundable €29 super-saving train ticket to Geneva from Venice will end up costing 232€ for the return journey for 4. And that's only to one place. To do any exploration in the Dolomites, go to Milan or Verona, Bolzano, etc the car will not only end up being more comfortable and flexible, but likely cheaper!

Make sure that "compact car that seats 4 in comfort" is big enough to carry all of the luggage, preferably in a way that the luggage is not visible from the outside.

You'll also have to buy a vignette for the drive in Switzerland. And unless you cross into Switzerland via the Stelvio Pass or somewhere west of the Dolomites, you'd probably traverse a part of Austria which also requires a vignette.

Unless you have one of the few premium credit cards, usual credit card CDW does not cover Italy. Read your fine prints.
Your fuel estimate of 15€ would be an underestimation especially for a car that seats 4 in comfort including cruise luggage.
Several of the cities you mentioned in Italy have ZTL and/or congestion charges. Breathtaking fines would follow you months after your trip based on images from silent video cameras that captured violations.

Wanted to visit Switzerland from Venice, but few people suggested on this forum that the Italian Alps (dolomites) region is a better better choice and very comparable to the popular and scenic beauty of Switzerland

So finally decided to try to Val Gerdana region either Selva or Ortisei for July 2016

>>>It ends up being about 35€ / day to rent a compact car that seats 4 in comfort. <<<

A compact car will not hold two suitcases. When I've rented compact cars in Italy (for only two people), one person's luggage had to go in the backseat.

You haven't included tolls or parking. As pointed out above, many cities have ZTL and you won't be able to drive into them. You will need to park on the perimeter and transport in. I believe you will also need a vignette for Switzerland.

Make sure your credit card insurance covers Italy as most don't.

The Dolomites are VERY different from the Swiss Alps (Valais, Bernese Oberland, etc.).
Both areas are fine, both have some spectacular places, but they are different!

Fuel costs for a drive from Venice through the Dolomites (Cortina and passes) to Val Gardena: 50-60 EUR