Marseille Airport is close enough to Avignon that you can travel between the two in as little as 38 minutes, though in most cases the travel time will be around an hour via TGV. The current schedule doesn't show a lot of trains, but that may be due to coronavirus-related reductions (I don't know). Flying into Marseille will probably mean an extra flight, plus there's the transfer time at (probably) CDG.
By comparison, the fastest trip from Paris-CDG to Avignon currently takes 3 hr. 23 min., but the trip can take over 5 hours. I'm guessing connections between CDG and Avignon are not very frequent even in normal times.
Which route is cheaper will depend on your origin airport and travel date as well as when you choose to buy the airline ticket. From my origin (Washington DC), Marseille is not usually a cheap destination.
Which route is faster will depend on how flight arrival times align with train departure times. I'd be a bit more concerned about switching from air to rail at CDG, because I think you'll find fewer train options there, which may make it trickier to figure out what train you can safely make.
You can check rail fares on the SNCF website.