Specsavers’ business model is essentially franchises combined with own brand frames.
The franchises will probably vary somewhat, but are unlikely to be significantly worse than a given small independent, and very possibly may be significantly better than a given small independent, since the Specsavers group will have minimum standards for its franchises to meet, and there will not be similar requirements imposed on an independent.
The big question therefore is probably whether the choice of Specsavers own brand frames is sufficient for you. For example, if you particularly want an Oakley frame (sports or otherwise), then Specsavers is unlikely to be the place to go.