Dolomites or Tuscan countryside?

First of all, if you haven't yet bought your plane tickets, it is better to do the reverse itinerary. That is fly into Venice and return from Rome. Venice is easier while you adjust to the different time zone. Also flying back out of Venice will mean very early flight and reaching the Venice airport from Venice takes longer (and more money) than reaching Rome airport from downtown Rome (a quick 30 min by train or taxi). Regarding Tuscany or Dolomites, it's hard to decide for you. If you have ever been to Northern California, the Dolomites are similar to Lake Tahoe/Yosemite/Mammooth Lakes area. The Minarets of California could be in the Dolomites. Tuscany is very similar to Sonoma County. Of course neither the Sierra Nevada nor Sonoma have those pretty centuries old villages perched on top of hills, but that description might help you make a decision.

I'm not sure you have time in 10 days for all you have listed. Rome - 3 nights Florence - 2/3 Dolomites - 3 ("few")
Venice - 2/3 This is a fast pace! 2 nights stops are not, IMHO, worth it. If you only have 10 days, narrow it down to 3 locations. I like the idea of seeing something other than cities. Maybe Rome/Tuscany-not-Florence/Venice. Read your guidebook and see what appeals to you in Tuscany. You could stay in a hill town like Montalcino or Montepulciano enjoy some quiet evenings and drive about the countryside during the days to see amazing scenery and tour other towns. June is early for the mountains. It's really quite cool up there. It's beautiful, but unless you hike, there's not much to do. Have a car for Tuscany, but take the train to your Tuscan location and then on to Venice. You'll enjoy the scenery more, I think. And it is indeed better to start in Venice and head to Rome last.

OK, I tried looking at this from a 18 year olds point of view... I think she would like Tuscany better- it offers more for a younger crowd. She will love the shopping in Siena and the weekend amazingly huge market they have there with great clothes and things. I noticed a younger vibe at the restaurants and just more hip. Dolomites are AMAZING!!!... if you like the outdoors. Hiking/Biking- otherwise Im not sure what else anyone would do there. And what that other poster said regarding the feeling of Austria- second that- you dont even think your in Italy.

The Dolomites are beautiful but you would have a more compact, reasonable trip traveling through theTuscan countryside on yor way between Rome and Venice ( or vice versa). The Dolomites, lying well north of Venice, would be out of your way. Another consideration is that the Dolomites area ( Sud Tirol) only recently became part of Italy. The area "feels" more like Austria than Italy, and in many places the first language is German rather than Italian. The food is distinctively Tirolian, and the villages are alpine in design,mwith chalet-style wooden buildings rather than the charming old stone of Tuscan hill towns.

Thank you all for your advice. We are set with airfare (lucky to use miles) so cannot change the route; however, it certainly sounds like Tuscany is the winner over the mountains.
So the question now is drive from Rome to Florence and see inbetween OR drive from Florence to Venice and see inbetween?

You might want to train to Orvieto. Tt's a beautiful hill town and interesting contrast to Rome. Then when you are ready to leave, you can pick up your car
there for your travels through Tuscany. The car rental agency is across from the train station.

I agree with picking the car up in Orvieto70 minutes by local train from Rome Termini. The brand of rental car across from the train station is Hertz, but also check with AutoEurope.com to see if they can arrange the rental cheaper than getting it thru Hertz.com. The roads up to Siena, Tuscany/Chianti and Florence are very easily driven from there.
And if time allows, see Civita, an incredible hill town southwest of Orvieto. It's well worth the short trip out there.

Again, many thanks for the guidance - everyone has really helped firm up our travel itinerary.