Which one should i use?
I've been reading the threads and can't follow them too clearly on whether 0.21 introduces issues/bugs that weren't there in 0.19, or whether the ones it solves that were in 0.19 out weigh the ones it introduces.

Any educated and experienced opinion much appreciated

Thanks..
Ian

Use version 0.21. Better yet, grab the version 0.70 of the Better AI mod, which includes the 0.21 unofficial patch and also improves several things about the AI:

http://forums.civfanatics.com/showthread.php?t=319673