I went from London to Barcelona and back by train. On the way down, a sleeper from Paris to somewhere like Perpignan, then a morning express from Perpignan to Barcelona, and on the way back, daytime TGV from Barcelona to Paris.

The return leg was far better.

Getting the sleeper train means hanging around in some dingy third-rate terminal waiting for it to leave. I didn't really sleep on the train, because even though they run it slowly, it's still pretty loud and bumpy. I shared a cabin with a well-behaved friend, so i wasn't even mixed in with strangers. Changing trains in rush hour was not fun. In the end, it took me long enough to recover in Barcelona that i could have stayed over in Paris, got a TGV that morning and not been far behind.

Whereas the TGV back was a TGV. Comfortable, nice scenery to look at, decent bar for a train.

If you sleep more heavily than i do, or if there is some real value in being at your destination earlier in the day, a sleeper might be a good choice. But in the age of high-speed rail, i don't think it makes much sense.