Is Shawwal fasting as valuable as Ramadan fasting?

There is no such thing. No supererogatory is like a fard. Shawwal fasting is a supererogatory act of worship. Ramadan fasting is obligatory. Yes, Shawwal fasting is blessed in terms of its virtues and rewards, it is a valuable opportunity, but it is never at the level of a fard or a substitute for a fard.