Sorry but haven't used both, can only comment on Google sheets.
My guess is they probably all include roughly the same feature set.
If I were evaluating that - my 2 concerns would be:
1) How close does the product come to behaving like Excel does. Why? Because the world is heavily attached to Excel. From your end-users "using" the sheet, to a developer designing it, the closer it aligns with all of the shortcuts, behavior, colors, matching, entering formulas, selecting cells - everything - the easier everyone's life will be, and the less they'll have to maintain TWO sets of skills, totally separate.
2) Does it allow scripting. Surely it won't be VBA, that would be too lucky? But does it allow scripting that is max easy for people to learn. Some like the fact that Google Script is "a lot like javascript", some don't...up to you and your developer base.
I guess I'll add.......Google sheets has MANY robust features to allow a live data connection (down to excel, text, access, anything), and many features to limit user access/permission to books, tabs, ranges, etc. Does Smartsheet have them too? Just to consider.