I'd grab 6.0 since you can get it cheap with a DV card. I doubt you'll find Premiere 5.1 cheaper then 6.0.
I know ErMaC is still using 5.1, I guess he just likes it better. However I believe 6 is the way to go, because you'll probably upgrade eventually and 5.1 & 6.0 are quite different in how they handle effects.
Just get what's cheaper and you can learn, they both are good
~klinky