Neither are that great; Swing is just as strong (read: EXACTLY AS STRONG) as the Sledge Heart, the rare badge drop/steal from the Chucklissa/Oholina/Hoohoolia/Teeheena bosses, and without the glitch to get multiple drops from bosses, means you will only get possibly ONE of if you opt for the Suit, and therefore only one badge with the "HP on Solo Hammer" effect badges; by opting for the Swing, though, up until the endgame, you will only have one available equip that has the "Auto-HP Heal" effect: the Wool Trousers.
However, all this being the case, neither are very valuable in the end; the Swing can be replaced by Sledge Heart, which can be gotten in multitudes, and the Piranha Suit is weaker than both the Wool Trousers and the B.Brand Jeans, making it useless in endgame... and since the Piranha Suit can only be gotten by a special method, it is usually required to be obtained to achieve "100% completion", despite being wholly inferior to its badge counterpart, which is merely a name-swap of another badge in-game, really...
I don't think I really get why they did this, honestly... it's cool and all, but rather stupid too, in the end, to make you pretty much have to choose between two otherwise missable gears... They should have made another Piranha Bean appear at endgame so you could get both, or another separate enemy to give Piranha Swing, making the Suit that much more valuable for completion's sake... or something... I think the completionist part of me is just kind of annoyed at this aspect. :/
''Sage'' of The Denpa Men board (Completionist Medal of Honor: http://i48.tinypic.com/20gmued.jpg) 3DS FC: 1590-4884-9269 | Etrian Odyssey 4: DEFINITELY F.O.E.