My R.O. in the concrete is 32"x21".

I can get a vinyl window 31-3/4 wide and install it directly into the concrete opening, sealing with a little spray foam and caulk, or I make and install a treated wood frame inside the R.O., and get a proportionally smaller window to fit inside the wood, which I think is a more traditional installation.

I'm not sure what the advantages/disadvantages might be of one or the other other than the fact that wood always fails eventually.

Your thoughts?