I own both and personally enjoy both very much. Very different games with some similarities in certain modes due to the programmer. Similar strategies as well as completing modes leads to very large bonuses. That's really where the similarities end tho. I think the tightness of the shots on guardians is a turn off for friends that come over causing it to be ignored more.
Stranger things, after being dialed in, is the house favorite by a mile. It just never gets old for anyone in the family. Totally legit separate strategies you can go for. Go for modes to stack the bonus or fight the demo for supers. Both valid strategies.
I've had an offer on my guardians and considering it but would never for stranger things and it's been here a year and a half verse 7 months for guardians.
Together they make a great package with the cleland audio updates. Rocking 80s tunes that complement each other nicely.