the carnac area is our favourite. weather is a bit better than further north. beg meil - it rained the whole time which has made us biased, but given the choice i would go to carnac.
you have the standing stones at carnac. golf du morbihan is pretty. men du beach is one of our top beaches anywhere - and we live by the coast and are fussy.
grande metairie have been to about 3 times. it will be busy but does have a lot of things to do sport wise and a good pool.
vanne and auray are lovely towns (parking is likely to be a problem perhaps).
if you get a detailed map you can find quiet beaches off the beaten track. there was one slightly north of carnac - long stretch of sand, with practically no-one there. we picnicked in a field of orchids.
quiberon - only been there off season as does have reputation for being busy. we found lots of other places and nice beaches to go to anyway so couldn't be bothered to return to this part.
the other campsite might be worth considering - i've not been there - and the pitches sound smaller. there have been reviews on here. but it does have the advantage of being walking distance to men du beach and the supermarket - if you are worried about traffic. just off the main road where its located there's a nice area with bars/restaurants - we missed this the first few times we visited
guites are really boring. even my 2 year old got bored.
We stayed in Morbihan last year so we were about an hour and a half from Quiberon, an hour from Carnac.
Didnt find there was an awful lot to do at Carnac, and I am big into history, but we have 2 dd's who were 2 and 1 so that may have jaded my view a little. Quiberon was wonderful, we only went for the day but the beach was fabulous. You can get a train from Auray (I think) which takes about an hour and cuts the traffic problem. Not sure I would go for a week but then we were in a gite not on a site. There are some wonderful beaches around there, and plenty to do so I would say go for it.
I'm sure someone more knowledgable will be along soon.
Over the last 2 years we have been to Village la Plage near Guilvinec and L'Atlantique near Beg Meil. We love Brittany and are wondering about venturing slightly further south to Carnac and Le Grande Metairie in 2013. We have loved the relative peace of the places we've been so far but the dc are 8 and 11 so I think will love the bigger site. I have read that the Carnac/Quiberon area will be much busier than where we've been so far so do feel a little anxious about that. Any reassurance from people who have been to both areas?