Popularity wise i would say the XRay (T2 or 007) is the most common in Sydney, dont know about other areas. The Cyclone is starting to catch up with XRay though. But there are also a lot of other good kits available.
I personally believe you should try to get a belt drive kit if possible. Generally most people find them easier to drive.
From what i have heard the Cyclone is easier to get going out of the box than the XRay however once you get the XRay how you like it, it is a very good car.
However, these are top of the line cars with the price tags to match.
The XRay T2R is a step down a good car but a little hard to get a hold off due to stock levels.
The Cyclone S and TA-05 are perhaps the next step down. Someone pointed out in a forum the other day that a lot of people with TA-05s end up buying heaps of hop up parts for them while with the Cyclone S this doesnt yet seem to be the case (although i have :P). However, there are a lot of hop ups available for the Cyclone S if you do choose to go down that path.
One main problem with the Cyclone S is get a spare pair of carbon graphite C Hubs, as the kit ones are very brittle. Its just C Hubs which seem to be a problem the rest of the car is OK
A couple of other things are look for parts supplies. It helps if you buy a kit which you can spares for easily.
And if you are going to race have a look at what the people at the track are racing. If they have the car you have, they are more likely to give you a hand and know how to help you with problems.