There are I'm sure miles of posts and threads on this one if you'd have done the search.....
....at the end of it, then it comes down to personal choice, price, and what suits you.
I've always used optimates on my bikes (currently using a mark 2 and a mark 3)...and never had a problem.
I connect mine directly to the battery, with the lead supplied, and this I then tuck into the tool compartment when not in use.
With the Bmw version it can be plugged directly into the accessory plug on the left side, under the seat area...so I guess the Bmw one scores on that front.
There are also concerns that the Optimate is not Canbus friendly...but it tends to be widely disputed....I'm sure a load of posts will prove/deny that statement.
My 2p's worth.....if I needed to buy one I'd go for an Optimate everytime.
I think the Accumen version always get rave reviews too...though made by Optimate I believe, but harder to source (normally stocked by car dealers).
But I contacted Optimate directly a few years ago, and of the two the technical department recommended the Otimate(3 at the time).
I should say that I keep mine on charge all the time it's not in use, and I don't have an alarm to put a drain on the battery.
Hope that helps?
Steve.