CUDA core is Nvidia only so you can only compare CUDA cores with Nvidia cards and the card with most CUDA cores is not necessarily the best choice.
Gbs is short for Gigabyte per second, which is a transfer rate. The higher, the better.
BITs are memory bandwidth. The higher, the better.
The best way to choose the best GPU is looking at up to date benchmarks. There are hundreds available on the web and Tom's Hardware does some really good benchmarkings and test. Look up Tom's Hardware's article "Best Graphic Cards for the Money". They do an article every month.