I've had both, but currently don't have either.......a decision I regret !
Both were fantastic varmint calibers...the Vartarg was probably inherently more accurate but with reloading both, probably not much in it.
Pro's of the 204...factory ammunition, bit faster, brass a bit more readily available.
Cons 204.....more powder needed to get the same speed.
Overall not too much between the two.....you can run the 204 a few hundred feet per second faster, but the big plus for the Vartarg is as Tom says, very efficient case that uses much less powder to get the same speed.
It's not all about speed though...or is it ?
My next 20cal will be either a 5/35Smc or a 20x47 because....Yes it is all about speed ! Lol
Regards, Rob.