It would be hard, maybe even impossible to precisely estimate the speed for either of them, but Piccolo seems to be more impressive.
Roshi was fast enough to have entire fight scene within one second, he exchanged punches and kicks, they were spiting on each other, then they spent some time thinking, than they had a short conversation, then they played a game etc. and Roshi did all this while he was still holding back. Normal people were unable to see their movements.
After being shot at, Roshi was able to turn around and catch 9 bullets.
Roshi was able to create many after images
Roshi moved his hand so fast it created a wind so strong it blowed away a 70-80kg heavy superhuman and smashed him into a wall.
Kid Goku (21. budokai) , and Namu who were both slower then Roshi were able to jump several km high in the air, a feat which requires a movement of legs much faster than the speed of sound.
Bora who was slower than Roshi was able to trow goku and yacharobe above clouds, which means he can move his hand faster than sound.
Tao Pai Pai was on pair with Roshi, or slightly faster, and he throw a stone pillar 2300 km away, a feat which proves massively hypersonic movement . (more precisely, the initial speed of that pillar must have been around mach 14, and he was faster than a pillar, so his speed must have been around mach 20. link and link , 2 links for you to check my words)
From all of this I think it is safe to assume that Roshi is much faster than the sound.
Goku 22nd budokai
Kid Goku on 22 budokai was so much faster than Roshi, that Roshi was unable to see him
Goku after mystic water
Then Goku drank mystic water and became many times faster than before.
Tien, who was on pair with Goku before he drank the water, was so slow he was unable to kick Drum, and Goku, after water, was so fast he speed blitzed him. He also became as fast as young king Piccolo, while before he was slower than even the old Piccolo.
And Tien was unable to see his movements.
Yet even this Goku, who speed-blitzed Drum, and was moving so fast Tien couldn't even see him, was still too slow to even touch mr Popo, and mr Popo was so fast Goku was unable to see his movements.
Goku 23rd budokai
23rd budokai Goku was so fast that even Kami, who was vastly superior both to Mr Popo and kid Goku, was unable to see him or Piccolo while they were fighting at top speed.
And Raditz was much faster than both Goku and Piccolo at the beginning of DBZ.
And saiyan saga Piccolo is much faster than Raditz.