The DDA series in some cases is a regular scope with a set of disk drive analysis software tools, and because they are less search-friendly, they're often cheaper.
That said, some DDA variants also forego the higher voltage ranges, so even with the 1M input, their frontends may not be able to handle as high of a voltage. Not a deal-breaker for some (and makes no difference if it's through an active probe), but it does also differ between DDA models, worth checking the datasheet.
Not totally sure on what a fair price would be, but the seller knocking off a bit over 8% going from 1100 to 1200 may seem like more of a discount than it is. If you think this is the scope for you, I think something like $900 is fair enough not to be just turned down, but would feel like a lot better of a deal to me. If you're more on the fence, something like $750 would still be a fair offer (low, but not so much as to be insulting if I were the seller), but which may net a better price. If you end up with the 6100A, I'd max out the motherboard's CPU and memory right off the bat, then upgrade the firmware (I think the most recent 32 bit is 6.something) - it will feel much more responsive that way.... and an IDE SSD would make another big difference but would be basically a drop-in replacement with the old disk's image.
Still, I think if you could manage to find a WP 7k for $1200, I'd take that one over a 6100A for $900, for whatever that means to you. It would definitely mean a waiting game, probably for months, but it is entirely possible to find one. The 6000 series was somewhat shortlived, and while 1GHz is a lot of bandwidth, I sort of have a tough time justifying the bench space for the full sized chassis for "only" 1GHz if you're dropping this kind of money into it. Very much to personal preference, setup, and finances, though.