I own the 8400 but considered the 990. Basically, the 990 is really super-duty 8400. I went with the 8400 because I was mowing only three acres on a weekly basis from April till November and I felt the 990 was overkill.
It turned out to be a good call because my ground is smooth and I take it easy. If I had more irregular ground and wanted to pound on the equipment more, the 990 would have been a better choice. The 990 will seriously outlast and outperform the 8400 but used prices for either one will be soft. The 990 is easier to adjust the depth of cut. Over time, you would likely buy two 8400s to equal one 990.
If I was doing 10 acres and wanted to go fast and the ground was rough and I had the $$$, I would probably go with the 990.