The Ultimate is the adjustable CF paddle that has been on the market for a while.
The Accelerator is the new Full CF paddle.
Comparisons to Ultimate:
The Accelerator is
>newer on the market
>lighter in weight
>non-adjustable
>more expensive
>has an oval grip as opposed to circular
>is composed with a carbon fibre sheath, finer quality of CF throughout the entire blade and shaft
>has been proven to be more 'injury prone' due to its somewhat less rigid design (I have been witness to a crack in one of these due to paddle clicking in the boat)
The following site only has info on the Ultimate:
http://www.apexcomposites.com/carbonfiberpaddles.html
But if you have any other specific questions, you can contact Mike Kerkmann mkerkmann@bellnet.ca for more information since he deals with both of them.
I personally own an Accelerator and I haven't found any issues with it. Sometimes it's a little hard to handle in REALLY tough winds, but other than that, it's a solid blade.