Neither! Either of those will all too often result in a convex face on whatever you're surface grinding. The down pressure causes the wheel material to "cup", and creates a convex face on whatever you grind. Can it be avoided with wheels that soft? Yes, but most people don't have the patience to take several passes, to remove .005" of material.
Personally, I use a urethane wheel, serrated, that is the equivalent of 100 durometer on my surface grinder.. and even then, trying to take too big of a "bite" on a pass, will result in a slightly convex surface.
Some of the best performing wheels, in terms of making/keeping the surface of the piece being ground flat and true, are solid wheels made of steel or aluminum. They are very unforgiving in the sense that they will not absorb bumps and lumps in a piece being ground, but if used correctly, you will not find a wheel that will produce a flatter, truer surface. As with most things we do, or most machines we use...there is a "give-n-take"..... it's dealer's choice on what you feel you can give up, to gain what you want. The ultimate goal of surface grinding anything, is to achieve the flattest, truest surface you can...... so whatever it takes to achieve that.... would be the way to go in my opinion.
The "compromise" wheel for me is the urethane I mentioned previously. My original came from South Africa, but I have also had the same wheel made by SunRay Corp. (search for SunRay contact wheels. Their a govt contractor, who make urethane wheels for tank tracks, but also make contact wheels "on the side". They have a few common ones, but most are strictly custom/made to order. They are very good, honest people...just be prepared for a 4-6 weeks wait, depending on their backlog)