In particular, on cello. I got the impression that both are strong (either by pressure or speed) at the beginning of the note, followed by quick release of pressure/speed. What is the difference between the two?
I assume you mean the difference between ^ and >. Both signs are used as accents in generally. The correct designations are marcato (>) and martellato (^).
Usually marcato means accentuated and martellato means strong accentuated. So you see that there is only a small difference. And it is often discussed how exactly these two accents are played differently and in particular with a certain instrument (like in your case a cello).
However sforzato and/or sforzando (sf and sfz) are dynamics. They are dynamics and mean with sudden emphasis in the sense of sudden change of the volume.
So summarizing: accents like ^ and > are similar to dynamic instructions like sforzando and sforzato, but they are not the same.
A sforzando(sfz) is the loudest and strongest accent. It is not really considered a dynamic as another answer suggests.