These are possible 3H bids, but are substantially different in
playing strength. If 3H is bid with both hands, the responder may have
to guess. Bidding 3C to show a 3H opening allows the responder to bid
3D if he wishes to invite 4H. This has the usual advantage of transfer bids, in that partner
becomes the declarer and the lead comes up to his possible tenace position. A
disadvantage is that it places less pressure on the opponents in the
bidding: Second hand will have two opportunities to act.
From The Official Encyclopedia of Bridge, Fifth edition