Generally quality wins. A site that offers good marshals, games, and play environment would be more likely to get my repeat custom. In the same way that a poor site would be less likely to get me to spend my time and money going back.
Some competition is healthy, sites can learn from other sites so that they improve their quality or offer a niche event, i.e. WW2 theme day.
Of course if one site costs £40 to play at and another only £20, then you may choose to play twice as often for the same money. Personally if the £20 site was not offering value in its gameplay, then I would choose the more expensive site.
Agree that too many players is not good, I like to have less people so that there is more room to manoeuvre. Games can always be adjusted to allow for less people.