Both cars you are asking about are good cars, and both can be fast as you can see in my signature i run the 966. I have built several mrx5 for friends and have drove it as well, it has its good and bad like any car does. What i would say to you is see which one is easiest to get parts for.
On another note the newest addition the te and now v2 have a big sweet spot for setups, i have a base setup i use for me that has been a good starting point for any surface i have run it on. I think the serpent is the most durable car i have raced with and that is a big part of racing.
You will not be disappointed with a Serpent 966!