Both companies are as good or as bad (depending on your point of view) as each other

Have you been with any other company before ???
How about DIY -- contacting the site direct and finding an offer from one of the ferries/tunnel direct

Can't comment on Canvas but have holidayed with Eurocamp for the last 12 years. Keycamp are part of the same group. Never had any problems whatsoever.

Most sites we've been on, and there have been plenty, also had Canvas holidays. Have spoken with people who have holidayed with them and I really can't see much difference.

As far as travel is concerened, St Malo is only about 25 miles away whereas Roscoff is more like 120.
That said, sailings into St Malo arrive early morning and you are unlikely to be able to use your accomodation before mid-afternoon. Some sites will allow you to park up but you will have a long day to fill. Roscoff would give you more options as there are more sailings each day and you could arrange for a ferry that would fit in with the arrival times.

When we went we travelled Poole-Cherbourg arriving in Cherbour late morning and on camp about 2.

Canvas say they won't let you in to your accommodation before 4 pm. We were at Des Ormes last summer and we got the breakfast time boat from Portsmouth to Caen and it was then a couple of hours drive, getting us to the site about 4. We came back from St Malo though.

Never had any problems whatsoever.

Me neither...

Eurocamp took the stress out of organising my family holiday in France.

Have no experience with Canvas, sorry, but I like the sound of madcaravanner's idea... Try contact the site direct... You could save yourself a pretty penny