Are you hiring a car ? If not Sirmione is probably the easier to reach as you can get a train from Verona to Sirmione /Desenzano. There isn`t a train service to Bardolino though I expect there is a local bus service.
Having visited both many times ( I live just a few hours drive away) I personally would plump for Bardolino not that I don't like Sirmione but it is a day tour destination for lots of bus tours so it can be very crowded during the day though it quitens down after about 5pm. Sirmione probably has more bars and restaurants and lots of little streets with shops, etc.
Bardolino is smaller and I think quieter. If you like to walk there is a lovely lakeside walk to the town of Garda from Bardolino which also has lots of little streets with shops, restaurants and bars.
Whichever place you decide to make your base there is an extensive network of boats that ply between all the towns on the lake so you don't need to stay in one place all of the time.

Thanks Judith, very helpful. I think we shall opt for Sirmione as it's easier to get to from Verona, esp as we are only there for 3 days.