Serve & Volley is more difficult to "learn" in my view. In order to become successful at S & V, you have to not only learn the various skills required to succeed in that style...but you have to acquire anticipation of where the opponent's return is going and move forward to intersect the ball. Learning to anticipate where an opponent's ball is going involves a lot of trial and error. You can't learn to anticipate simply from being taught..you have to learn from experience...and that means getting passed...often. It's a learning process that is humiliating and discouraging for any player as you are being essentially being relegated to doing something that will inevitably result in failure more often than not...but is part of the learning process.
Most junior players I know have developed strong baseline games and can succeed reasonably well amongst their peers by exchanging ground-strokes. However, the moment you try to teach them how to S & V to introduce more versatility into their game...they inevitably falter and balk at being relegated to a losing proposition. Only a few have the skill, mind-set, and determination to persevere in learning a new style. From my experience and observation, S & V is definitely more difficult to learn for the simple fact that it is a skill that is typically taught AFTER someone has already developed a baseline game and players are less inclined to learn a new skill that has a high learning curve.
"Speak softly but carry a big stick; you will go far"
- Theodore Roosevelt