All of them are great cars - Though they tend to be the "bastard children" of the Porsche community, they combine the best of both worlds Good handling, good speed and a bit more affordable than the 911 series. Arguably, you can say that maintenance is a bit easier on the 911, and overall cost of ownership is lowest on the 914 series.
This link below will give you a general list of what to look for whatever you decide to get. Short answer, you will enjoy any model you buy, they are all very fun cars to drive. As a general rule of thumb, turbo's and the S series cost more to maintain, and are a little more difficult to repair.
http://www.pelicanparts.com/techarticles/944_buyers_guide/944_buyers_guide.htm
Welcome to the dark side, we'll teach you the secret handshake when you buy one.
AFJuvat