Hey Ted, I think the safe route for you would be the RC-100. It's tried and true in this setup, I'd hate for you to run into trouble down the road because of unforseen problems with the SR. I'm sure it's a great machine in it's proper environment. And a big ol yes on the torque, I did not know there was that much difference. Almost double and these heads would show that. Make SURE you get some type of forestry package when buying, I know it will add cost but is easier on you to add into the financing than go out of pocket.
As far as heads the timberax cuts the best to me bar none. But that being said they do not deal with rocks. If you are rough with a timberax the blades could go dule in as little as an hour causing it to not grab material well at all. In my opinion you would be better off with an carbide of some sort in rocky conditions. Plus it would be less down time for you also,{not sharpening}
I do not have any experience with a magnum head so you'll have to feel out that one with a demo yourself. I've heard of vibration problems with them so just demo to check for yourself. And a good deal is a good deal. When your talking close to 20k every little bit helps.
As far as finished product you can always reprocess the material, and honestly unless your dealing with someone who has had this done before I don't think chip size is going to be a huge concern. That can be easily felt out be talking to the customer. I just do alot of residental work that is close to homes and therefore they are a little pickier and so am I.
As far as how close to the ground you can get, Regardless of teeth/blades the skids on the head will limit how close you can get to the ground. Most heads have different height setting and will allow lite 'grading' but not the rotor tilling effect your thinking of. But a carbide will do that where a timberax shouldn't for blade's sake.
Yes 500 an acre would be a good starting point, it leaves a cushion for your cost and the customer. That is about what I charge if its a solid acre, my jobs rarely consist of solid acreage. Usually alittle here or a row or two here or a trail there. Do yourself a favor and buy a measuring wheel from northern or somewhere to protect your interest. I believe an acre is 43,000 or so sq. ft.
I hope your trade-in goes in your favor and you have you an RC-100 soon. Keep me posted. Robbie