Is it preferable to use celery or potatoes for karpas? Or is something else preferable altogether?
Both are fine.
It has to be a vegetable of some kind, and according to some Rishonim it should be similar to Maror, so that there might be some preference for celery.
However, the general custom is to use any vegetable, including potatoes.
See Tosafos Pesachim 114; Rikvah in commentary to Haggaddah.