Hi Daniel,
If you ask a vest designer for an opinion on that subject, you'll get a biased one, but here we go!
Wherever you connect the arm to the vest, the weight of the Steadicam is still out front, so let's first rule out any differential there. Connecting the arm to the back of the vest does not magically transfer the weight into your back.
Although the Exovest can be front or rear mounted, I use mine almost exclusively front mounted. The exception is when I'm using a 3rd arm section. The extra section has to go somewhere, and rear mounting provides the perfect solution of creating space.
The main difference in feel between front and rear mounting, in this case (as it's the only one I'm really familiar with) is that there's less torque bending your body forward, but that's because you now have some extra counterbalance weight out back. That might feel nice, but I'd rather build up a little back muscle than subject my hip, knee, and ankle joints to carrying around the extra weight that rear mounting entails.
Apart from the extra weight, there's the extra girth to deal with too. You're always going to be wider with a back mount, and there are often going to be situations when that's a liability.
However, and most importantly, comfort and confidence in the equipment is paramount. Whatever works for you is the right system.
All the best,
Chris
Chris