Wow, I've never heard of this Feldenkrais thing until now. From what I can tell it looks very interesting.

I've done yoga for a more than a few years and have always enjoyed it. I've found that right after I am calmer and have a sort of relaxed "noodly" feeling to my muscles. It's very nice.
I've taken only one tai chi class and thoroughly enjoyed it. I've been trying to find more classes but can't find any that are cheap/free.

ITunes is a great resource for classes by the way. For yoga I use the 20 Minute Yoga Sessions from YogaDownload.com. They're long enough that I get the benefit but don't have to fight to find an hour to set aside to for it. I've also found some Feldenkrais classes that I'm going to try (there's also some on Youtube). Unfortunately all the Tai Chi classes on there seem to cost mula.

I would say give a try for all three and figure out what you like. Of course you don't have to choose one. You will probably benefit the most if you go to an actual class (I attended many yoga classes to learn the basics before moving to in-home practice). But if you just want to try these things out online classes might not be a bad way to try them out.