The train by far for London to Paris if you are going from city centre to city centre. Getting to LHR from London is a about £3.50ish by tube to £15 on the heathrow express plus about 8€ from CDG on the RER if I remember correctly plus the lost time traveling between the airports.
Regarding the easyJet versus Ryanair take the first. easyJet are geared more for business people (ie flying from the main airports not strips in the middle of nowhere), crew is friendly, baggage allowance, flexibility and service is higher. Ryanair's philosophy comes down to "you paid a quid so don't expect anything more".
Whilst a little biased the following video does show all the issues and dangers of flying with Ryanair.
http://video.google.com/videoplay?do...83901290988541