There are probably a lot of different opinions but if you can afford it, go for the dry suit for cold water diving. While you can be reasonably warm in a heavy wetsuit (e.g., 7mm) after the initial shot of cold water, dry suits are nice because you don't freeze your tail off as much after the dive (being wet in cold air is far worse to me than being in the cold water in the first place) and will also keep you warmer between dives when you do more than one. The trade-off, of course, is the cost of a dry suit, a bit of extra training, a bit of maintenance, ... but you may find it worth it.
A good way to find out if you can hack it in the wetsuit is to get your OW cert wearing one. I wouldn't invest in either until after you get that done.
Hope this helps.