How many terminals an airport has is (mostly) irrelevant, as long as your flights arrive and depart from the same terminal - which flights on the same airline very often do, but you should check to make sure.
Many airport websites have information about the minimum legal connection times for particular pairs of connecting flights. (I know Heathrow does, not sure about the others.) That will give you an idea of how much time you have to spare. For example, if you have 90 minutes between flights and the minimum connection time is 80 minutes, then you need to count on everything going just right. But if the minimum legal time is 60 minutes, then you have a bit more breathing room.
All else being equal, I like to choose a connecting city that's as close as possible to my destination city. My reasoning is that if I do miss my connection, there will likely be more options for alternative transport (including trains, buses, and other flights) to get me where I'm going.
Passport control to enter the Schengen zone is rarely an onerous process. A quick stamp, maybe a couple of questions, and you're through. The time spent waiting in line can vary from airport to airport and from day to day, but it's not usually terribly long, and it's built into the airport's minimum connection time.
You didn't ask about this, but if you're facing a similar choice on your return trip, be aware that it's a whole different ball game. If you're connecting from Frankfurt to Dulles to Newark, for example, you need to have enough time to go through passport control in Dulles (which can involve a long wait), claim your checked luggage, carry it through customs, recheck it for your connecting flight, and then go back through security (often the most tedious part, because most people have forgotten that they have to do it, so they're all rummaging through their carry-ons to get rid of their water bottles and the like). That can take hours.