I may have a chance to visit both of these parks in late Dec. to Jan. this year. I know there are several people here that regularly visit or live in the area. I've never been in India at all, and really don't know what to expect for photo opportunities during this time frame. Or the weather for that matter.
I need to look close at my equipment later on, as my go to lens is the 600 and I don't think I can handle that from the back of an elephant.
I have been a few times, and am certainly no expert on the weather but as you are outside of the monsoon season you should be OK. There is absolutely no reason or need for an Elephant ride at either park in particular Kaziranga where in common with all other parks you can use the "worlds most uncomfortable" game viewing game vehicle the Maruti Gypsy.
The 600 will be perfectly adequate, but I would recommend something smaller especially for Elephant! I took a 70-200 + a 500mm.
Kaziringha was our favourite, although Tiger are very difficult to see there, you should see plenty of Asian Rhino a wonderful experience, the park should also be a lot quieter than most, largely as a result of it being a long way away, although the new road that is being built may change that.
Kanha is definitely more scenic than Bandhavgargh and slightly less of a "madhouse", with reasonable chances for Tiger.
If I make the trip I will be pretty much be going along for the ride, and will photograph what I can as I go. I don't have enough information on the area yet, but I think the trip will be worth doing even with all the unknowns. And I still have months to research the area.
I am so wanting to make that trip. I am on pins and needles concerning Burma and Thailand.
If the road opens in the next few years I will be there in a heartbeat.
If not may have to take the tour.