Well the vm20 is nice because that main jet swap is so easy. That's a big carb for the stock bike. If you want to go with that carb, why don't you get a pipe first. maybe start with an 80ish jet in the stock carb. Get that tuned in and when you feel ready for the next step - go with the bigger carb. vm18 is nice too but doesn't have the easy main jet swap.
The pipe will get you much more speed initially than a new carb. (edited)