I've been in drift boats since I was 4 years old and rowing one on my own since 14 in everything from flat, lazy water, to class 4 rapids....it's been a lot of years.
While drift boats may look similar there are quite a few differences depending on what type of fishing you're doing and the type of water you're floating. I might choose a clackacraft if I was just fly fishing but if I was gear fishing, back bouncing bait or plugs, I'd probably go with the Willie simply because of the hard chines they tend to track easier on the backstrokes against the current. Clackacraft has tried to remedy this with their tunnel and gulf stream bottoms but I still feel the hard chines on a metal boat work better in that regard but they can get you in trouble if you hit a hard eddy at high speed at the wrong angle.
Fiberglass boats stay warmer in the winter and cooler in the summer than metal boats. For the most part you can customize a metal boat more than a fiberglass boat. Fiberglass will "slide" over rocks on their own while you will want to coat or put uhmw on the bottom of a metal boat.
As far as brands go I've had Willie, Kofler, and Clackacraft. They were all great and each had their advantages. You really can't go wrong but if I was fly fishing out of the boat instead of just using it for transportation I'd more than likely go with the clackacraft because it's more "rounded" inside and fly lines tend not to get caught up on as much stuff. For customization and creature comforts I'd definitely go with a metal boat like a Willies or Koffler.
I've rowed a Pavati, and while nice, just felt like a tug boat to me. It wasn't as nimble as the others but if you're not doing anything technical that's not much of a worry.
If this is your first drift boat, buy used. Put some dents in it, bang it off some rocks, slide it down some banks, and use it hard. After that, if you still want another one you'll have a better idea on what you truly want and need.
If you have any specific questions feel free to ask