Like some of the other guys I have both. I use the tiller mainly around the house and the discs are kept at the farm. I have brought the tiller over to the farm for getting the vegetable garden ready etc., but usually I just use one of the discs. As a general rule I would use the tiller for an acre or maybe two at the very most. At the farm I'm usually trying to smooth out more than that after the hogs have torn it up. I prefer to use the tiller on the little B2400 as I think it makes a much better one pass job. The discs go on bigger tractors and for me, I have to make at least four passes from different directions to make an acceptable job whereas one slow pass with the tiller is usually acceptable. Speedwise, the disc will probably be a lot faster especially as you get up to around two acres.
I don't think the R4's will be a handicap with a disc unless it's really wet and slippy and you probably shouldn't be out there with it anyway.
Cowboy has it right. The cost of a good, heavy duty disc is about half the cost of a good, heavy duty tiller.
I don't know if that will be any help but it's all I've got as I've never used a harrow. I did use a section of chain link fence once just to smooth out after the disc.