It sounds like you want to keep things pretty open, I would go with a Railpass.  As far as basing yourself out of a city, I think it really depends on where teh day trip is to.  If its half way to the next city, just keep moving.  If its kinda out of the way, base out of another city.  I think your itenerary is doable.  I like at least 3 or 4 days in a big city to get the feel of it and it looks like you’ve got that covered with some time left ovre for daytrips.
have fun!

Use www.railsaver.com to see which, if any, railpass is the best deal for you.  Follow the “I have a good idea of my itinerary route” and click “I prefer purchasing railpasses (to point to point tickets) ‘only when it saves money.’”

When traveling by train, I like the “base city/day trips” mode.  I do not like to pack up and move to a new city every other day.  Sometimes we take an overnight trip from a base city — traveling light and leaving the majority of gear behind and sometimes we plan an all-day or single night “stopover” between two cities where we’re going to spend more time.  Occasionally we rent a car for one or two days to explore.  While you can always accomplish getting somewhere without a car, you can sometimes see and do more in rural areas if you’re not at the mercy of bus and train schedules. 

You could stay in more remote areas and visit central cities as day trips and you might save some money on lodging.  We did that in Netherlands — other cities and towns are only minutes from Amsterdam and the trains run every few minutes. 

I’d suggest reading Europe by Eurail.  It’s based on the base city/day trip concept.  See if your library has a copy and don’t worry about getting the current edition — there are very few changes from year to year.

Daytrips from a base can turn out to be pricey and time consuming depending on your destinations and where you’re starting from.  It just depends.  Sometimes it makes sense, sometimes not.

On a more general note, for a first trip, and with 7 weeks’ travel time, you seem overly focused on southern Europe. You’ll probably get a better feel for the continent and for the south if you actually spend a little time in the north as well.  Mareseilles and Nice aren’t really “France”.  Suggest you schedule maybe 10 days seeing Paris and Amsterdam, or maybe a bit of Germany or Switzerland for perspective.

I’d just tell you to be careful; southern Europe is notorious for being like the pickpocket capital of the universe. On my last trip I was only in northern Europe where it’s not quite as big of a problem, so I felt like I got acclimated better; I can almost guarantee you if I went to Italy as my first European destination I would’ve had something stolen. Not trying to scare you or anything, just saying It’s really a learn as you go kind of thing. We could sit and tell you what to do all day, but until you do it yourself, you just don’t know.
 
I’d second the notion of you sort of branching out a bit more too. I think you have plenty of time in your itinerary; if you’re gonna be in France, you may as well swing through Paris, and check out some bits of Germany, too. I think otherwise, everything might start looking “the same” after a while! And that’s the last thing you want

Day trips have the advantage of having to pack up your stuff one less time, but they’re costly, as you have to include the price of return travel (and you can waste time doing this). Also, in general the cities tend to be more expensive, so when staying in the smaller places you have the benefit of cheaper accommodation, meals and drinking. I find the nightlife in smaller places can be even more fun than the cities, as it’s harder to get lost, easier to get home and you tend to meet more locals, which after hanging out with fellow travellers can be refreshing (not that I’m at all suggesting hanging out with fellow travellers isn’t fun – it is, but isn’t the point of heading overseas to meet people from different cultures?)