Chuckatuck,
I wanted R4s, but since it was used, I took what it came with. My neighbor who sold it to me moved up to a DX55 and got R4s with his. I'm not sure it will make that much difference as far as preventing ruts, but R4s will probably be better as far as traction in the wet. I have bogged down twice in the marshy area when I couldn't get out with 4wd. However I was able to pull myself out using the FEL which I always keep on, even when mowing.
I always drive over the septic field whenever I mow and I always mow with the FEL attached as I stated above. Total weight is about 3 tons. My septic field is in one of the dryer areas of my property as I suspect it is with yours.
The DX29 plus mower and without FEL is probably about 3000 pounds vs. 3800 for the DX35, but you have smaller tires and ground profile so the psi are not all that different as someone else pointed out. If you are going to get bogged down with the DX35, chances are the lighter DX29 won't make a difference. For the price, I would definitely get the DX35. It is way more tractor.