Well, as mentioned before, it depends on the tank you want.
Q.R.F. has an interesting choice of WWI vehicles, but the casting quality (particularly with the older ones of that range) is sometimes very disappointing, so expect much grinding, bending, filling and... cursing.

The Minifigs Mk. IV comes as either as
bull (male) or
bitch (female) variant. Minifigs vehicles are quite cleanly cast, but require some dry-fit before glueing. I would recommend them.
AFAIK Peter Pigs Mk. IV tanks are made of resin with some metal parts. The downside is that there's only a male variant. And it is a bit pricey compared to the Minifigs one. What I don't like is the modeled-on base. But that's just me. I base my vehicles only if there's no other way.
Hope this helps.