I think 2T's are longer and slimmer, more for an older or potty trained child. Whereas the 24mos are cut shorter and fuller, more for a younger baby.
My DD will be two next month and is only in 12mos. It is VERY frustrating finding clothes for her. Everything looks like it's for an infant, especially now that she's going potty. Almost everything has snaps at the crotch or is very short with a matching diaper cover. She can almost wear some 2T's while 24mos are WAY too big around the waist.