Omg, I had a club tournament and I wondered about this, because I played a first round on clay and then for the second round they asked me to play on the less used synthetic grass courts because no clay court was available to play on at the moment (many tight first round matches I suppose, or maybe I just wasn't relevant enough
). After that, I played quarters on clay again!
From personal experience, I would say that either way is tricky, but clay to grass is slightly more difficult.
Point number one, the ball bounce. Both types of court have irregular ball bounce patterns, but come on, grass wins the award of most crazy bounces (clay usually only has irregularities when the court doesn't have enough maintenance or when the ball hits the borders of the lines, because unlike grass, where the lines are painted, the lines of clay courts are seperate entities from the court, creating a slight elevation).
What is difficult to get used to in these bounces is the way that the ball bullets off after hitting the ground, something which is easier to notice when you hit good slices or drop shots.
Second point, the movement. A person who is used to sliding on clay will suddenly feel like a cow on grass. Your reach and court coverage will diminish drastically if you were previously used to playing on clay.
What happened to me was that in my second round match I had a lot of trouble adapting to the ball bounce on grass. Preparation was so difficult that at one point I gave up on groundstrokes and just sliced and volleyed my way to victory. When I came back to clay, my movement was fine, proving that grass wasn't a setback on that area, but oh my god, the groundstrokes... My timing was completely unregulated by then, but I eventually realized that instead of waiting for the ball to bullet away off the ground, I had to go meet the ball, striking it much more early than in grass. So there you have it. You are completely bored by now, If you're even still reading
, but I just wanted to show how I think that clay-to-grass transition is more difficult, all this from a personal perspective.