I have both and use them of Red Fox and the European brown rabbit we have downunder.
For Fox`s I was using using the 17 with 20 V-Max at 4000fps and was getting a bit of Splashing so next winter I will be moving up to the 25 V-Max.
Wirth the 20 vartarg I use 32 gr V-Max at around 3750 fps.
My suggestion is to get two rifles built up exactly the same.Same stock, barrel,action ,trigger and scope.One chambered in the 20 VT the other in 17 Fireball.After a season of calling report back and tell us which one you prefer.
When I go out they both come with me.