I'd go to France. It just seems so much easier to get on the bike and do some miles in France. Far less worry about which roads will be suitable to cycle on and whether traffic will be unpleasant.
You probably already know this but La Rochelle is also a great area for cycling. Head down to La Tremblade and follow the cycle routes through the dunes of the Cote Sauvage, for rolling country side head inland to Surgeres, for spectacular skies, wildlife and history (and maybe some wind) roll across the marshes to Brouage, if you want to shelter from the heat and any wind head in to the Marais Poitevin. I haven't done it yet, but after having cycled to the Ile d'Oleron I really want to visit the Il de Re by bike as well.
The country around La Rochelle will suit you training needs better, you'll want more miles rather than hard miles and it can only be another plus that it's almost impossible to have a cafe stop that doesn't involve propping the bike against an Abbaye and basking in a medieval town square.
