These are both short Vega trades, meaning that they benefit from volatility lowering, however, the structure is different and the pros and cons of each are different.
Which is Better?
The Iron Condor would be better than the more narrow strike Iron Butterfly. The of using an Iron Condor is that when it does go against you, it is more difficult to repair and/or you can lose more money because you took in less premium, by selling options that were further from the money.
Overall, though, it does have a good probability of profit greater than that of the Iron Butterfly.
The Iron Butterfly is also a trade that benefits from lowering volatility. It is structured by selling an at-the-money call vertical and an at-the-money put vertical with varying long wing widths.
The Iron Butterfly has more narrow structures than the Iron Condor, however, it has a risk-to-reward, because your return can be so much higher on-the-money at risk than with the Iron Condor.
This is because you received more premium selling the at-the-money options. Because it has this greater risk/reward, the Iron Butterfly can be put on in a wider range of markets, both lower volatility and higher volatility.
Even though it is short volatility, it still performs well, even in lower volatility markets because of the risk reward.
Of course, both of these trades, require that the price stay inside of a range for the trade to be profitable. The Iron Condor gives you more room and the Iron Butterfly gives you less room for the price to roam. However, overall in most markets, I preferred the Iron Butterfly, because of the increase risk reward.