I definitely believe it is better to have Aer Lingus independent in order to keep Ryanair on its toes.
I am not going to fly with Aer Lingus if I can help it, given the price difference, but think that them being there is better for the consumer.
The main differences between Ryanair and Aer Lingus are:
Aer Lingus offer pre-allocated seats. As these are for sale it increases the chance of not sitting with the people you are travelling with.
The Aer Lingus seats recline whereas Ryanair's do not.
Aer Lingus cabin crew use the word "purchase" whereas Ryanair use "buy". On Aer Lingus you'll be asked "Would you like to purchase a drink from the bar" while you are trying to ignore the cabin crew and read your paper.
I have a Travel Money Card from the UK Post Office which I can top up with Euros to use a Visa Electron and not have to pay credit card charges on Ryanair.
Ryanair have really opened the whole of Europe up to the likes of me. I've been to Slovakia, Slovenia, Latvia, Germany, France, England, Scotland, Spain, Austria, Poland, Holland all using Ryanair.
I love it.