iDBUS looks more comfortable than Eurolines as it is advertised. You can view pictures and videos of the bus features by searching on YouTube.
iDBUS fares, in the absolute, are slightly higher than Eurolines ones and remain the same for every seat of the same bus i.e. they will not vary as the bus is filling up. You can find them on the iDBUS website.
Examples of iDBUS comfort features:
- Power outlets
- Longer distance between seats, meaning more legroom
- It is possible to move an aisle seat sideways towards the aisle so that there is less elbow fight with the window seat passenger.
iDBUS pickup and drop-off point is the Bercy railway station in Paris. A metro station is present and the Lyon station is close, from which all the high-speed trains serving southeast France depart.
Eurolines use its coach terminal on the east side of Paris, stuck to the belt freeway and one of the freeways that goes north. It is further away from the city centre and a metro station is available too. The fact that the station is directly located on the freeways makes that the bus does not need extra time to run in city streets.
There is also a third option, Megabus: can be very cheap, starting at £1, although there may be only few seats at this price. The Paris drop-off point is Porte Maillot. There is a metro station in the area. Pickup is done at a minimal bus stop on the street.
Note that Paris to London is a very long trip when done by bus. There is extra time needed to board a ferry and cross the Channel. For instance, iDBUS offers a night trip, departing in the evening and arriving in the morning.
According to their Q/A, though, most iDBUSes cross through the tunnel, which is faster.
I would strongly advise you to take Eurostar if you catch fares that are suitable for your budget.