to start, no specialist at all, here...
The answer really depends on how many "if's" apply to your situation:
* if (re)starting from scratch as an operator, A220 might be the more modern, 'better' plane
* if you are a current Airbus (only) airline -> A220
* if you are a current E17*/9* operator -> E2:
- existing logistics/working relation with Embraer
- minimal crew conversion training costs (pilots 2,5days - no full flight sim required)
- a (limited) degree of commonality between the E-family reduces somewhat the spare parts requirements
* if looking for the higher end seating capacity -> A220
* if looking for the lower end seating -> E2 (for the really lower capacity wait 2 yrs. for the E175E2)
* if wanting to maximising the pax/cabin crew ratio -> E2
* if looking for an efficient replacement for your A319 -> A220
* if looking for max. range -> A220 (although it seems Embraer is 'downplaying' the real E2 range)
* if aiming for LCY, Toronto's City, or similar short runway, steep approaches -> A220-100
* if you have no strong commercial bargaining position: E2 should be the cheaper option
* wanting to take my preference into account -> E2: I prefer the 2+2 seating

Bottom line: academic discussion, look what fits best for your operations. So, one airline in Belgium knows the E2 is the better product for them.

My 'Future' scale: CS100/300: mediocre / A220: bright / Embraer E-family: good / Boeing-Embraer: better