A couple other considerations for using FTM as your primary.
1) Consistency of Place Names. You can control consistency of place name spellings and completeness better in FTM.
2) Filters. In my surname study, I use filters many times during the day trying to zero in on candidates to either place in my tree, or match from my tree. I understand the on-line version doesn't have filters. For example, who was born here, died there, or lived over there.
3) Relationship Calculator - does on-line have this?
4) Birth Date calculator - I use this in FTM all the time. Does on-line have this?
5) Fact Notes. I put details about facts into Fact Notes, including transcriptions of wills, census records, birth and death records, obituaries, etc. I understand On-line won't accept FTM's Fact Notes and doesn't have anything comparable.
There's probably more advantages of using FTM over on-line, but that's should be enough to convince you to use FTM as your main engine.