It's more fun to watch the mountain stages, but any stage is fun, and a unique experience. You won't get to see much of the actual race - even in the mountains, they fly by pretty quickly. One tip might be to find someone with a portable TV, or a bar/cafe with a TV tuned to the race, so that it's possible to see them go by, then rush over to the TV to catch the rest of the race. I recall watching the world championships in Verona like that - one guy had a portable TV in his car, and about 30 people crowded around it after the racers went by each lap.
The Bergamo stage looks like a more fun one - it's got some good climbs in it, and has a "strappetto" (short steep climb) towards the end in Bergamo - where perhaps someone in a break will try and take off on their own.
Which is part of why it's so fun... it's a great atmosphere!