Both options are just as viable, but the thing is that defence is something every character needs, while for offence you just need like 2 primary damage dealers. Defence sometimes feels less dependent on rng, which is why rng dependent defensive units are not looked upon all that well.

To me it really comes down to easier vs safer when wanting to clear runs.

A team who each do 1 million damage but have 100% damage reduction will always win fights but it will take forever.

A team who each do 100 million damage but have 100k defence have a chance of losing but will win events much faster.
Like obviously, something like Red zone omega is much easier to work around because we have a timer for the super attack, and in that case someone who hits like a freight train works better than someone who is a stone wall. But at the same time enemies like cell max can double supers, so it is almost a necessity to bring a character who can tank at least one super because it is not very likely to beat cell max in one turn. Like you can live without bringing a character with an apt above 30 million, but you definitely need a defensive unit who can either dodge or tank cell max. So either way, it is debatable and can work better or worse for different events and situations.

I mean not like it matters , most units do both nowadays anyway.