Nantes and Rennes Peripherique

I just follow the road signs - though timid folk get the heeby- jeebies by the high bridge on the seaward side of Nantes.

PS.  There are often bottlenecks and traffic delays on both those ring roads but a decent sat nav will give you clues to choose which way round to select.

I travelled back from Les Sables-d'Olonne to Calais last October via both Nantes and Rennes using the easterly route. 

Both were trouble-free (on a Sunday) but there were extensive roadworks affecting Nantes, which will probably still be there 9 months later.

... but the views are amazing!

I've have done the journey around Rennes and Nantes dozens of times, albeit in a car and from St Malo rather than Cherbourg.

Rennes is a moot point, given that you are coming from Cherbourg, and maybe the eastern route is better (from St Malo only the west side is sensible). 

Nantes I always did the west side; except once only, on a return journey, when either I was not paying attention, or I relied on signs, I can't recall which now, and I ended up doing the east side and I am certain it took longer and had more traffic so I never used it again.

The first time I saw the bridge, mentioned above, I admit to getting a bit nervous, but I am not good with heights (to put it mildly), but once done I was OK.  

Stopped for a comfort break at a very large super store of the N844 south of Nantes, took a wrong turn coming out and ended up in the centre in rush hour with a fully loaded motorbike on wet cobbles. All I could hear from my wife was unrepeatable.