One would need to have a detailed look at both to see what specific differences exist. However, basically, the two will have much the same capabilities. The main differences likely will be -

(a) slide rule side. No difference other than what particular equations might be incorporated in cutouts. The basic CD slide rule scales will be the same other than for specific index marks which a given manufacturer elected to include.

(b) wind rose side. The slide computer runs the wind calculation directly as the wind triangle is drawn. The circular computer employs a bit of simple trigonometry involving a secondary triangle but the result is much the same.

Which should you use ? Personal preference wins.

The circular units I have seen tend to have some high speed flight pertinent cutouts on the slide rule side which may make that style of more use if you fly in the flight levels. Hence, probably more use if you are intending to do the ATPL exams. Plus, the smaller diameter units have the advantage that they fit nicely into a shirt pocket. Downside to the plastic construction is heat and warping.

The slide style units usually are made of aluminium and, as a result, are reasonably robust.

Either should be just fine for PPL and CPL level studies.

Probably best to be able to use both, I'd suggest.