Digitals obviously cause your batteries to dump faster and they cost quite a bit. For most, they are excessive for the throttle servo. Locally, a lot of people like the JR 4750 because it is coreless (great for the throttle return spring which you better have... since the servo is coreless, it doesn't take as much tension form the spring to fully return the carb to neutral), reasonably fast and has more than enough torque. The JR return policy is pretty good (and has a 3 year warranty I believe) and it has been pretty reliable.
I mean, it really depends on what is available to you, and what is your preference. I'd say get a coreless servo, with at least 80 oz. of torque and a speed less than .15 seconds. Get a servo on the slower side if you aren't the best driver at the track... get a faster servo if you feel you will actually notice the difference (obviously, be realistic). There are many models out there that meet this criterea, so your choices are quite open. But everyone's got their own opinion so talk about this to your hobby shop, or use the search feature on this site... you won't have to wait so long for more information.