plastic for me - although had a built in aluminum one time that was fine.
I guess if new and you keep water out it wouldn't matter, but I have nightmares still over all the troubles we had with the first boat when I
was a kid - metal tanks with rust in the bottom, always carburator problems!
My last boat had plastic portable tank, I always left the vent open and
would just lay something over it if I got caught out in the rain - but it
stayed under a shed at home.