Awesome choices OP! I ride a Monster 795... essentially it is a mashup of the M696 and M796... it has the 696's frame and has the 796's engine, dash, exhaust system etc. you get the idea. Monster 795 and 796 are very similar and handles similar as well. The difference would be the seat height, M796 is slightly higher, has a single sided swing arm and larger rear tire.
Haven't ridden an FZ6, I have ridden a friend's 2012 FZ1 however, and I can tell you by experience that the FZ1's torque is very similar to my monster. The difference being the FZ1's power delivery is very linear and progressive kinda' like you have to work for it a bit, got bored with inline 4s very quickly. In contrast, the M795's power delivery is very raw, the power is given to you right away, and for me this is very fun, exhilarating and makes me feel very much alive! That engine is just delicious!
I have also ridden a friend's Monster 797+, there is a significant difference compared to the Monster 795. First is the seating position. M797 is very upright and "standard" seating position. M795 is sporty and a bit aggressive, you're leaned over a bit and your feet are slightly raised. Seat height - M797 is a bit higher, and there is more room for your bum to move around. Engine and power wise - on paper, M797 has less power compared the M796/M795. I'd say in stock form - yes it is, but after we changed my buddy's exhaust and air filter it felt the same to the M795's power. The delivery and engine sound however is very different, M797 is much more refined - the engine character is still there, don't worry, it's just more "controllable" I'd say on the lower RPMs. The engine and exhaust note is different as well since the cam profiles were refined too, also M797's engine is similar to the Scramblers so this would apply to that lineup as well in case you'd want to take a look at those as well.
About the Monster 821 haven't tried it, one the guys in our group has one though and he said that 821's "powerband is up top" so I'm assuming you'd have to rev it a bit higher as well. That's all I can say about that.
As for the maintenance... yes it can be expensive - if you bring it to a dealer, and it would be very similar to the japanese brands as well. You can save a lot of money if you can do the basic maintenance on your ducati though. Monsters are relatively easy to work on - I do all of the maintenance on mine.
Test ride the bikes that you are targeting if you can, you'll know which one to get, that's for sure!
At the end of the day though I still love my M795, there's just something about older ducatis that appeals to me.
Anyway, I hope this would somehow help you in your decision, good luck!