Mt Ventoux, is actually harder IMHO, and it has a certain cache that L'alpe D'huez doesn't (only people more into cycling know about Ventoux). For what it's worth, If you're going to be in Provence, I'd ride Ventoux, enjoy the countryside in Provence, and maybe go to the Côte d' Azur, rather than spend the time to go to the Alps just to ride L'Alpe d Huez. As pointed out above, its a bit of a trip from Provence to the Alps. If you have the time to spend some time in the Alps it would be worth it. But just to say I rode L'Alpe de Huez, I'd spend my time in Provence. Also the weather in Provence in April will be nicer than in the Alps, where it's likely to still be cold.