Yet u/TheScotchEngineer alleges
Higher IV is preferable, but by far the bigger factor for SPY verticals is delta, rather than Vega. At the relatively tight widths (often 2.5-5 points wide) of the spreads, IV has even smaller impact since the two legs offset each other on Vega.
Credit spreads give the benefit of SPY trading sideways as well as in the favoured direction, whereas debit spreads must move in the required direction, and within a set timeframe since theta decays the option.
Why would you desire higher IV for Vertical Credit Spreads?
I quote the definition of IV in Zvi Bodie, Kane, Marcus's Investments (2018 11 edn). p 718.
In fact, market participants often give the option-valuation problem a different twist. Rather than calculating a Black-Scholes option value for a given stock’s standard deviation, they ask instead: What standard deviation would be necessary for the option price that I observe to be consistent with the Black-Scholes formula? This is called the implied volatility of the option, the volatility level for the stock implied by the option price. Investors can then judge whether they think the actual stock standard deviation exceeds the implied volatility. If it does, the option is considered a good buy; if actual volatility seems greater than the implied volatility, its fair price would exceed the observed price.
Another variation is to compare two options on the same stock with equal expiration dates but different exercise prices. The option with the higher implied volatility would be considered relatively expensive, because a higher standard deviation is required to justify its price. The analyst might consider buying the option with the lower implied volatility and writing the option with the higher implied volatility.