One of the first decisions is the model to be purchased. In addition to different frame sizes there exists multiple alternatives and variations with different frame geometries to fit most needs. Basically all the carbon fibre frames made by the different chinese manufacturers seem to be variants of the same basic designs. Both road and time trial design are available. Most people seem to opt for the classic carbon road frame designs, but so called “aero” (as in dynamic) designs are gaining ground fast.
There are several other variables that need to be taken into account when picking a frame. Some of the models come with a stiff integrated seatpost. This means that the frame itself needs to be cut down to fit the rider. The benefit of having non-integrated seat post is easier adjustment and resellability. The bottom brackets can either be classical english threaded or more modern BB30 variant. Just remember your chainset will need to be compatible. There are different headset size alternatives as well.
Some of the frames have internally routed rear brake cable, some “aero” models even have internal cabling for mechanical front and back derailleurs. There are already some model variants available that enable using electrical derailleurs (eg. Shimano DI2 or Campy EPS). It is just a matter of time before all frames produced will be compatible.
Some of the models seem to have some resemblance to bikes offered by brand name companies. This has lead to speculation that either the same manufacturers provide or have provided frames for these leading bicycle manufacturers. While these rumors probably are not true it is obvious that very similar molds are being used and it is practically impossible to distinguish between frames from leading brands and knock-offs by naked eye. Some say that even test riding the differences are negligible, but as always you get what you pay for.