Faced with M10 or BH40 choice, I would turn to M10 in a heartbeat when used with a light weight tripod. I like to carry my tripod on a shoulder strap when walking about, and too heavy of a head mades the tripod carry with the head more downward than I find comfortable.
Yes...I want a "light" ballhead. But I'm confused about what you're saying--according to the respective company's information, the BH40 is 494 grams to 498 for the M10 (including their clamps). Essentially the same.
And, I think the BH40 may be "shorter," too, by over three quarters of an inch. There's not much for me to base my decision on with these--they both seem to be top quality. It'll probably come down to a decision on the day I get the legs.
Went to try out the 2530 today at a local store. Still don't have the 2531. And no 2540's in stock. So, not much of a worthwhile visit!

