I've ran both, only 1 system of each, but this is what I noticed. Tekin handles loads better IMO the RX8 is a great esc, and the T8 Line of motors stay very cool. I also like the hotwire app from tekin better than the castle link. The one thing I don't like about tekin is their 90 day warranty. I have had a tekin fail on me but it was after hitting a large curb at about 50mph with my lst2, and it broke almost all my plastic parts in the front end, and tore my aluminum motor mount from TD-racing.
it also depends on what you are racing. If you are doing blinky, theres a ton of escs you can use with little to no issue on buggys, and STs. But if you are running 1/8th I would say tekin all the way. I have the T8 1550KV truggy motor, and was running it with a 34T pinion and a 64T spur, and I barely would hit the 130 deg F mark, after a full pack.
Most people here locally run LRP ESCs for blinky, and some even for mod. But My next 1/10th scale esc and motor combo will be a tekin RS pro esc with a Reedy Sonic motor. The Reedy Sonic motors can take some insane amounts of timing, and still stay relatively cool.
(I just noticed you said its for a 1/8th scale mugen. Get the tekin you won't be dissapointed. When racing, the dual mode that the tekin escs have really come in handy. Smooth acceleration, and great torque and speed.)