I shopped these two tractors several years ago, and decided on a
B2920. It ended up costing only a few hundred dollars more than the Deere, but was a much more substantial tractor with more ground clearance and bigger tires. For rough terrain, those things matter a lot.
Your prices seem a little high, so I suspect you haven't gotten an actual price quote from a dealer yet. Normally, you can get Kubotas and Deeres for 10-15% off MSRP depending on the market and the dealer.
The R4 tires are OK on the grass as long as it's not soft/wet and you're in 2WD (and I should note that even turf tires on my smaller mower would tear up the grass under those conditions). Overall, for loader and backhoe work, I think the R4s are a good choice as a compromise for mowing.
I didn't mow with my 2920 (have a small Deere riding mower for that) but many people mow with them and I have yet to hear a complaint posted here. We would certainly hear about it if there were problems.
The only other knocks against the Deere to me was the extensive use of plastic, and the cheap fuel gauge on the fender. Neither of those probably matter in the long run, but when I sat on the Kubota and felt the metal fenders and hood, and saw the proper fuel gauge in the instrument cluster, it just felt a lot nicer to me. Personal thing I suppose.