If both offer the same route MAD-LGW and the price difference is what you state, pick Ryanair any day!!! They will both charge you to check in a bag, they will both charge you for everything up in the air. So, the difference comes to checkin in, as Easyjet will let you get to the airport without a boarding pass, and print you one for free and Ryanair will not. I am at the moment an avid user of Ryanair on MAD-STN, as in August I did MAD-LGW with Easyjet and August, September, october, november and december, I flew and will be flying Ryanair on MAD-STN and MAD-LGW as they price it cheaper. For me, any extra saving goes to the hotel in London, as I can book a really nice hotel with Hilton Visa Signature promo (75 pounds including buffet breakfast) and enjoy two or three days at the City. Some other times, I just conect to other Ryanair destinations to visit friends.
Pick the cheaper and enjoy the savings in UK. Seat prices in an airline on short and medium haul, work as a commodity being sold. You just buy the cheaper one, as their is almost no difference, because all of the airlines will take you where you want to go on time and safely.