I wouldn't necessarily say "better", but the butterflies do make most of their profit in the last week. If you do trade them in the last week or so then you have to consider whether a butterfly actually benefits you with it's 3 sets of commissions and slippage both ways as opposed to just a straight call/put!?
there is no free lunch. if there is, you won't be the first to discover it. the risk-reward ratio is always there, the more reward, the higher risk. very few seasoned trader would take a fly into expiration (unless it is DITM.) when you are close to expiration, you are faced with pin risk. http://www.investopedia.com/terms/p/pinrisk.asp if you got assigned, you have to scramble to cover your shorts because one of your long leg is ITM, while the other is OTM. Nothing that is unmanageable, just a bit messy when you have other things on your mind during the busy expiration week.
There is a tradeoff between how long before expiration you put the fly on and the maximum reward you are likely to get from the fly. If you put a fly on just before expiration, the risk/reward is not as good as if you had put it on with several weeks to go. Yes, commission can be an issue with the number of legs traded (4 not 3) but comparing butterflies with straight calls or puts is meaningless IMO - different intent. Slippage is a bit of a red herring, especially if you are trading a delta neutral flies on a liquid products. Pin risk is good to know but is not an issue depending on how you trade the fly. A PUT fly, a CALL fly and an IRON fly with the same strikes are all synthetically equivalent. I personally prefer to trade flies more than 14 days, usually around 21 days, and up to 30 days out. Though there are always exceptions and it depends how I arrive at the completed fly position. Flies are cheaper the further out you go. Flies are cheaper the higher the IV. IV decreasing has the same effect on the position (any position) as time moving forward. A neutral fly position is helped by time passing and IV decreasing. Therefore, putting on a fly close to expiration doesn't leave much room or time for either to happen and thus risk/reward is not as attractive. Generally I will roll/close short options before expiration - that includes flies. MoMoney