It’s easier to get going. The analogy I’ve seen used for this is apple vs android. Instantly has everything you need out of the box to run campaigns at scale. Smartlead requires a bit more tinkering to set things up
It’s best to have a dedicated sending tool while keeping apollo for list building only. both instantly and smartlead are great. If you’re going to utilize apis heavily, then I’d go with smartlead, if not instantly works.