They probably have a slight edge in terms of versatility (though remember Flamini can play fullback), but in terms of straight personnel there is nothing in it - certainly not enough for Wenger to be considered negligent and Mourinho not.
Arsenal established defenders:
Debauchy
Mertesacker
Kosceilny
Gibbs
Monreal
Chelsea established defenders:
Ivanovic
Terry
Cahill
Azpilicueta
Filipe Luiz
Arsenal non established defenders
Bellerin
Chambers
Chelsea non established defenders
Nathan Ake
Kurt Zouma
He'll often send out a team with a specific purpose which has got a load of stick for in the past.
Wenger's idea of tactics is his usual 72-75 min subs.
Or he used to dabble with bringing on Cygan.
Though this year the subs are getting later and later.
Yes they are numerically equal, more experienced and perhaps more versatile as you suggest, but tactically quite different.
The defensive aspect of football goes beyond the four players across the back. Chelsea’s defence is afforded protection by arguably the best signing in the Premier League this calendar year, behind them is possibly the best keeper in the league.
They appear to be a fantastically balanced team at the moment, this may change.
When they need to they will bolster the midfield with Mikel or Ramires, both players able to perform unselfish roles. Unlikely to catch either in the position for example where over half our team were yesterday at 1-0.
..... With a small one as well