travelling from Milan to Venice

Sure. You won't have a lot of time, but plan on it taking two hours from Milan to Verona and another two from Verona to Venice. Padua would be in a similar window, only a longer trip from Milan and a shorter connection from there to Venice.

It's only about 3 hours by train from Milan to Venice. It should be easy to stop off in Verona, look around, then catch a later train and arrive in Venice mid-afternoon. I'm not familiar with the Padua connection. Check at the Trenitalia website.

THanks guys. Any preference for Verona or Padua?

Padua: Scrovegni Chapel (reserve ahead), market squares, Basilica of St. Anthony, and more. Verona: Roman Arena, Roman theater, Castelvecchio, and more.

It can depend on the train. We bought a 1st class ticket for a Regional train (slowest of the bunch) from Venice to Verona. Most of the Regional trains took 2 hrs, although there was one that was only 90 minutes, which was the one we took.
As far as tourism goes, I can only speak for Verona. But you could get a decent amount of the usual stops done in several hours. We walked from the train station into the city center, which took about 10-15 minutes. It took us a few of that to just get our bearings straight as to which way to go. You could use the RS book for sites to see. There are several within a relatively short distance.
Once you exit the station, take a right and follow in that direction. You will have to cross the street and veer left a couple of blocks down, and then head straight to the colosseum. Or you could bus it as Rick's Verona day-trip states.
There are ESCity trains that are only 75 minutes from Verona to Venice San Lucia, but they cost more.

In Verona there is a bus stand in front of the train station; buses leave frequently for the city center. Verona has enough of interest to occupy a few hours. Between the two cities I would probably choose Padua for its cathedral and ambience. I went there on a tour so don't know about train-bus connections. The Scrovegni (sp) chapel experience I would not care to repeat. There is a long wait getting in (they want you there an hour before your reservation time with no place to sit), then time spent "acclimatizing" in a closed in room, then the few minutes in the chapel (in our case there was a small child screaming the entire time in the chapel), all while half the chapel was covered with scaffolding. And I didn't find it so exceptional compared to all the other art and stained glass in Italy. The area around the cathedral in Padua is very nice.

If you plan on breaking up your train journey, a word of caution. If you begin out of Milan on the higher-speed trains (ES, EC, ICPlus, etc.), you will need seat reservations. Just don't buy a ticket from Milano to Venezia Santa Lucia and think you can get off for a few hours and then get back on some later high-speed. You'll most likely need to buy a separate ticket for each leg. For the Padua to Venezia Santa Lucia leg, you can easily just buy a regional ticket (R). No reservations permitted on an R train. It's the local - you just get on. This gives you the option of getting on when you want as most trains heading eastbound to Venice will stop in Padua. They run frequently.

Ive been to both and preferr Verona.