I agree with a lot of that - and touching on the problems/errors you point out - I put that down to the way we asked them to play rather than any serious problem with either players defensive abilities, though would agree on Mason being more of a attacking centre mid than a box 2 box type. Bentaleb however could play that role and even a more defensive role IMV and could see him stepping in for Dier at times this season and performing well.
Agreed. I think Bentaleb would look a lot better defensively with Alderweireld and an in form Walker behind him, and without Townsend, Paulinho, Adebayor etc ahead of him, and with a more settled team after a full season under Pochettino behind them. The work rate from a "front 4" of Kane, Eriksen, Lamela and either Dembele or Alli depending on the day is so much higher and so much more structured than some of what Mason and Bentaleb had to contend with last season I really don't see how it doesn't make direct comparisons between the performances of the deep two midfielders very difficult.
It's interesting that the comparison keeps being made between Bentaleb/Mason and Dier. No doubt Dier is better defensively than Mason, Alli, Dembele and probably Bentaleb at the moment by the looks of things. Mason has looked good in a deeper midfield position against top teams in the past. Against Arsenal Alli was quickly moved to a more attacking role with Dembele moving deeper to make us more solid. Not really a criticism of Alli as it was his first huge game and he's only 19, but it's worth keeping in mind that similar things were true about Bentaleb and Mason (to an extent) in games last season.
Both Mason and Bentaleb have played more attacking roles for the youth team than their respective roles last season. If we assume that Dier is currently the first (central) midfielder on the team sheeet and Eriksen and Lamela have two of the attacking midfield roles locked in the current competition for the other places is between Alli and Dembele (currently holding them) and Mason, Bentaleb, Son and Chadli (when fit).
Right now those places are Alli's and Dembele's "to lose". But I would expect those other players to at least put heavy pressure on them when they get the chance to play. As Mason, imo, already has done. And I would expect Dier to at least have to keep up his very high level of performance to keep Bentaleb behind him too. Son's place in the team looked locked in before his injury and Mason started 4 in a row earlier this season, when we were performing well. Bentaleb looked pretty much an automatic starter this summer. These things change rather quickly.
None of that should be seen as me belittling any of the players currently starting just to be clear (as accusations have been thrown around in the past), it's just that I rate Mason, Bentaleb and Son (as well as Chadli) very highly.