Well you really can't go wrong I mean it is a RB that your choosing. However your choice might be determined more by the tracks you run on. If you don't have any tracks where top end is a real factor then an S3 or S5 (good short tech track engines) will be good purchases. Now on a long open track a WS7 or a C6 would be the ticket. Why not a C6 with the price drop on them thats a pretty good buy now.
http://www.amainhobbies.com/product_...ducts_id/23020
with the coupon its another 10% off making it about $329. Great deal on an enginge that was $100 more just a month ago. Not to mention you will never lack power. One of the few engines that can be really geared up to calm it down. Putting that unusable excess power to good use on the top end.