Fat chance at getting six DIMMs to run anywhere near DDR3-2500.
Anyway, faster memory is faster than slower memory, right up until you need more memory than you've got.
If you have a program, or programs, that uses even 13GiB of RAM, then 24GiB DDR3-800 will be faster than 12GiB DDR3-2500.
the 12 gigs would be better in all apps that use 12 or less just because its faster but obviously if you have a program that uses 13 or more (like blameless explained) the 24 gigs
the amount of gigs the ram has doesn't really make it faster
if i was playing a game on 4 gigs and one on 8 gigs and they both have the same speed they would have the same performance because the game uses less than 4 gigs