I would get the 125b, if you ever want something more juicy later, you already have the esc. Also, the 125b gives you some head room, should run cooler too.

in terms of runtimes. assuming the same gearing, how will a 7xl with 14 cells compare to a 8xl with 14 ?

i assume that the 7xl will be drawing more current and therefore have a lower runtime?

that is correct