I have a V800 at home and work for an organization with an 11000 XL.
The main selling point of the 12000 XL (and earlier ones in the series) is that they can scan oversize (11"x17"/A3) material. They're a common sight at archives for this reason, but they're necessarily huge machines. If you need to scan items larger than 8" x 10"/A4 and can afford it then the 12000 XL is really the best (in 2022 only?) choice. On a scanner like the V800/V850 you'd have to scan the oversize item in two parts and then stitch them together in Photoshop, which is extremely tedious and not always easy to do correctly.
Image quality-wise the XL's are quite good, but not as good as the V850. And the XL lacks Digital ICE, which of course the V850 has.
If you don't absolutely need oversize capability save some money and desk space and go for the V850.