Vitamin B12 - Methylcobalamin is the active form of Vitamin B12
Vitamin B12 - Cyanocobalamin requires an enzyme in the gut to change it to the active form therefore if you haven't the "intrinsic factor" you MAY be wasting your money.
It's helpful to absorb some in the oral cavity as sometime the digestive system doesn't absorb it.
That study Briffa was talking about the guys taking an ordinary multi that contained ordinary cheapo B12 (Cyanocobalamin) actually didn't do any better with extra B12. so if you've already got B12 in a multi it MAY NOT improve matters taking an extra amount.
Do make sure you are getting plenty of CHOLINE in your diet.
Choline-rich diet tied to sharper memory
and I'm sure I don't have to remind anyone low vitamin d status also is linked to the mild cognitive impairment which often precedes dementia.