I recently priced a Glasgow to London weekend visit.
By far the cheapest option was Ryanair - Prestwick to Stansted. But of course that leaves you with a Stansted Express trip into town, and Prestwick is very convenient for some, less convenient for others (like any airport).
Next cheapest, but a fair bit more expensive, was Easyjet - Glasgow to Luton. Again the drawback was the Luton to central London trip.
Next up was BA - Glasgow to Heathrow. For a whole £4 more than Easyjet! (Approx £88) But to a more central airport.
The BMI Glasgow to Heathrow flight was much more expensive.
So, it pays to explore all the options!
It strikes me that Easyjet has more to lose than Ryanair in the predicted competition 'bloodbath', since Easyjet is getting squeezed from both sides - as they compete on many main airport routes and therefore are going head to head with the older flag carriers, whereas Ryanair operating to secondary airports tend to have less direct competition.
I'd imagine Ryanair will tend to pick up the truly price sensitive customers, whereas the 'suits' might look at the options I found and consider £4 extra for a Heathrow flight money well spent.