Some sweaters are better done in pieces and some are better done in the round. Arans and other intricately textured sweaters are better done in pieces because you have to keep track of wrong side rows, etc.
Fair isles and colorwork (except intarsia) are better done in the round, because you always see the pattern.
Lace sweaters need to be done in pieces because you need that seam to keep them from twisting.
Any other simple sweater could be done either way.