Paying French Motorway Tolls

[Samcat]

Looking for a bit of advice; I've driven in France many times but always avoided the Toll roads. This year we are heading from St.Malo to the Loire Valley, and will probably use the toll road once outside Brittany (no toll roads there). I was wondering which is the best way to pay? just cough up the €'s when asked, or get one of those windscreen tag things, and pay later.
Do the tags make it any easier/quicker?
Ta.

[Samcat]

Thank you all for your responses.