I'd begin with the featured game because it gives each reward its character. The number of spins matters too, but Starburst's expanding wilds and Gates of Olympus's tumbling symbols offer very different experiences. Shelbywin1 explains that the relevant code is entered in the cashier when making the corresponding deposit. In its UK free-spin package at
https://shelbywin1.co.uk/, SHELBY1 provides 200 spins on Starburst, followed by 150 on Gates of Olympus, 100 on Big Bass Bonanza and 50 on The Dog House through later codes. The £20 minimum for each stage is another useful part of the comparison. I'd also check the validity and wagering conditions before activating an offer. Seeing the code, selected game and applicable terms together keeps the reward structure straightforward.