Personally I wouldn't go for any Poncho that wasn't made from a breathable fabric. But they are very hard to find. I would avoid any non moisture permeable vapour MVP material for any personal clothing and that even includes gaiters. To keep warm you need to keep dry. Non MVP material Poncho might be OK for a short rain shower i.e less than 10-15 minutes whilst doing nothing else except stand or sit around.

There are some Survival Optimists around who think they can spend a night out in freezing sub zero temperatures with a cheap PVC poncho and one of those $1 Mylar survival blanket. They are usually found the next morning as frozen stiffs.