Townsley. If you can possibly afford it I recommend spending as much as possible on your steering servo. I use a sanwa/airtronics 94359Z which is superb. All the other big manufacturers make a top end servo and I'm sure they're as good too.
If you really must keep the cost down then look at the towerpro 955's and bluebirds.
Somewhere in the middle falls the Hitec 945MG but I can't vouch for any of these as I haven't tried them.

Mark Pyke. The low speed needle controls the mixture at and just above idle, after that the high speed needle takes over.
Obviously to have a great tune both must be set for good acceleration and good mid-top speed running.