Coin toss.
The tekin is sensored, which would appeal to racers or ROAR legal racers I suppose, though you are never driving at less than 1mph during a race so I dont know how much value that feature has really.
Castle has the better warrenty (90 day vs 1 year).
Castle esc and motors are tried, tested and perfected- tekin is pretty much an unknown, though tekin stands for top quality so it aint exactly gonna dissapoint I wouldnt have thought.
Assuming the tekin motors perform okay (not seen too many specs listed for the 1/8 stuff yet), then it should make for an interesting comparison; be a guinee pig for us....