Digital is better...more precise. The S3151 has two bearings and bushings vs S3004 sigle bearing. Both use plastic gears. Check the specs torque, speed, weight etc. to compare. S9451 even better.
I think you get what you pay for with servo's. I have had great luck with the S3004's. Several of my planes have them. The other reason is a bit extra for each servo adds up. I found I need metal gears for my rudders after stripping several out. High torque and speed for 3-D.
If money grew on trees all of my planes would sport digital metal geared high speed and torque servo's but at over $100.0O mine get what the plane suggest or better.
I have a 6EXA. My flying buddy has the 7CAF. I would rather have his. It is much nicer for the reasons Felix said and many more. Those did not exist when I bought mine and the 9CAP was out of reach. Do a search for complaints. My friend loves his.