the RBS is more IV sensative and time decay is a killer.its not a good idea for prolong holding,better to open when you expect a rise in IV and as soon it happens with a small move-to close it. much better solution for me is OTM calendar put and OTM calendar call in ratio 2:1-it imitates the RBS ,but with positive theta. RBS is good if you want to build up a position around the spread,depending of the move of the underline-you can covert it as broken wing butterfly or do a dinamic hedging around it. but to wait for a huge move with a 2:1 RBS is a too risky game...... BRS is the spread with the biggest gamma attack,balanced with biggest vega and theta exposure. you can use it as a lottery ticket before expiration.why? before exp.,options have almost no time value-the relation between gamma vs theta/vega is in your favor.
The only valid part of the above is the RBS performs well if IV increases but not necessarily better than the long put. In terms of a comparison of a long put to a RBS, you can draw no conclusions because it depends on the ratio, the IV and the time remainig. You can set up examples which will go either way. To obtain the OP's 2:1 zero cost RBS, that requiress higher IV and/or further out in time. Because the RBS has an OTM long leg, the effect of time and IV is diminished on it compared to the higher strike. If you isolate all but time, decay favors the RBS. If you isolate all but IV change, it can favor either side. The short answer is that IT DEPENDS on the situation/inputs.
You can easily find the answers you are looking for by trying the different cases using the Options Lab. http://www.TheOptionsLab.com It is always hard to evaluate a ratio spread without any example or using any software.
------------------------------------------------------------------ Quote from ramaTrade: It is always hard to evaluate a ratio spread without any example or using any software. ------------------------------------------------------------------ Expiration values are easy to calc but before that, I think it's a must to have something that takes multiple legs and depicts a P&L curve at various prices and time.