I use to have the 2 speed titanium with Brompton and now I have the 3 speed hub on another Brompton. I would say, the the 2 speed is okay IF you have the proper gearing. If you like going fast, then you might not be happy with the 2 speed because I was never really able to maintain a fast speed and always wished I had another gear to shift into. The 3 speed is the same as the 2 speed except you have the 3rd gear to go into. I use the one gear for take off and uphill, middle for cruising and the last gear when I am going fast or downhill. I still wish I had gotten the 6 speed so maybe I'll buy another Brompton with a 6 speed!
The titanium model is well worth it if your going to lift it often to catch a train or bus. You should also change the pedals to MKS pedals and grips to Ergon, which will lighten the bike and make it more stable at faster speeds. Good luck!