NO, this movie wouldn't have been "better" with Sean in it.
If Connery had played Bond in OHMSS, I don't think it would have been as good as it is the way Lazenby made it.
1- Lazenby was nervous and inexperienced, and it shows. That's a good thing considering that Bond, in that particular movie, is human, weak and in love.
2- They cast Diana Rigg to balance the inexperience of Lazenby. With Connery available for the film, it would habe been somebody else. And that defenitly would have altered the quality.
3- Connery's Bond would not fit in a weak and scared Bond (can you imagine Connery shouting for fear at the sight of a fake polar bear ???).
4- Blofeld is supposed to recognize Bond 'cause he already met him in YOLT. He doesn't, but it's Ok because we're having 2 different actors there (Savalas and Lazenby Vs Connery and Pleasance).
If Connery would have returned, the sequence would have been very weird.