What are your dates, actual or approximate?
Google for special events during your potential date range, especially in the towns you plan to sleep in, but it's a good idea to do it for all the places you want to visit. Festivals and big-deal markets (as in L'Isle-sur-la-Sorgue) can affect availability and rates for hotels. I don't make my reservations very far in advance, and last year I was unable to find anything affordable in Arles--and had great difficulty in Avignon--due to something going on in Arles at some point during June. I was told by my hotel in Nice that the Cannes Film Festival doesn't have much impact on Nice hotels, but it was my impression that it contributed to ticket-purchasing delays at the Nice Ville train station (no problem actually getting seats on the trains). I'm sure the film festival markedly increases demand for hotels closer to Cannes.
In general terms July and August are the peak season along the Riviera, with June markedly less busy unless there is something special going on--which there often is, because folks working in the tourist industry are not fools and try to create reasons for people to visit outside the peak period. I'm guessing that Provence works similarly, but it is just a guess. If your trip is likely to be partly in June and partly in July and you anticipate more difficulties with tourist load in some places than in others (it usually matters more in small towns), you might try to visit the challenging places in June.
Another timing factor is the markets. A lot of tourists really enjoy outdoor markets. They are often held only one or two days a week, and putting together a trip that allows you to see a few of them is a bit like working a jigsaw puzzle. Saturday is a particularly big day for markets, so you might take special pains to spend your Friday nights in towns with big markets. L'Isle-sur-la-Sorgue's largest market is on Sunday; the smaller one is on Thursday, I think.
Then there's the weather. Last year the weather along the Riviera was coolish the first half of May then warmed up rapidly. I imagine that is fairly typical. If you expect to be outdoors (but not on the beach) more in some destinations than others, you might try to hit the outdoor destinations early. Nice is an example of a place where you might be indoors more than average, because it has a lot of museums. The smaller towns often have just one key museum each (or none in some cases).
I have not been through either the Marseille or the Montpellier airport. The cities are quite different. Montpellier is a university city with a large (medieval?) historic district. Marseille is a more rough-and-ready port city. I didn't love Marseille, though I thought I would. I made the mistake of staying near the St. Charles railroad station and found myself with no interesting places to eat for many blocks; that colored my view of the whole city. If you stay in Marseille, I highly recommend that you choose lodgings closer to the port in the touristy area. Location is not as critical in Montpellier, but you still might want to be in the historic area, which is where most of the restaurants are located.
If you mean that the trip would be longer if you fly into Montpellier because it's farther from Nice, go with Montpellier. More time in France is always good.
