If both models are used, which one is most common? Specifically, on Thursday departures from Marseille?
They're operated by Class 373 trains: either a refurbished e300 or classic Eurostar. You can check out which train is used by searching for a route on CaptainTrain.com. The Marseille train is consistently shown to be the e300:
I've found a guide on how to get this information from Seat61.com:
In fact, it's now even easier to tell whether you'll get an e320 using www.captaintrain.com. In the search results, the train number will begin e320 if it's a new e320, or e300 if it is either a refurbished e300 or classic Eurostar.