The Braille 21 lb battery is a Deka ETX30L powersports battery that costs about half what Braille charges. I ran one in my E36M3 for about 12 months, including winter, before it would no longer start the car. But the E90M3 is a bigger motor with higher compression and more electrical junk to wear down the battery. I would not bother with the Deka 21 b battery unless you don't mind regularly trickle charging it, live in a warm area, and don't mind short life. I now run the Mini battery in my E36M3 (which BMW supposedly runs in its E46M3 CSL and E90M3 GTS) and it is doing fine in its second winter. I would not go any smaller than that on my E90M3 since I drive it year round including winter and it sits outside for days at a time sometimes.
The Voltphreaks lithium ion stuff is nice, but LI does not do well in the zero degree weather I see sometimes. It is also too expensive for me. Great for a race car with a large budget, though.