[BaldyDave]

Optimate 4 is tried, tested and frequently recommended on this forum, so that more than anything else would make me buy Optimate. I've had an Optimate 4 canbus edition for 4 years now and it's kept my 9T's battery in good nick and plugs into the accessory socket without fuss.

No idea about the BMW charger, but I'm going to guess it'll be more expensive and probably not offer more than the Optimate does...

 

[BaldyDave]

Can't say I've had any issues leaving it connected to the bike for a few months at a time.