Well I think whatever you feel the best choice is. If you're not throwing it around carelessly, you don't need a metal mount and the plastic feel could be gotten used to. If it's cheaper to go with the Mark II version, I'd say go for.
If noise doesn't disturb you and you are picky about build quality, keep it.
I found one of these as well at a local camera shop. I picked mine up used for around $100 and really like it so far. I am still considering the 1.4 just for the full time manual focus, but I would be hard pressed to think it would take better pictures. This is a great lens at this price, and it does feel much more substantial than the plastic 1.8.