I put 3/8 copper tubing in mine because I had an immersion chiller I could cannibalize. I regret it - I really wish I had 1/2 inch in it. all my other fittings are wide open 1/2" - camocks, valves, etc. That coil really slows the flow down - not so much that my system doesn't work well, but enough to bother me.
I don't think the convoluted coil makes enough of a difference that it's worth getting - from the reading I have done.
if I replace my coil I will go with 1/2" tubing. go with the stainless