When I joined the industry I had no idea how unstable it was. On average I have not had the same type, base, employer or a mix of the three for more than 3 years out of nearly 20. If you choose to come in to this game you are going to need to have a very understanding and very flexible partner who needs to be as informed and involved in your decisions as you are. That is the only way it can work. If you can accept that and importantly so can your partner then the Ryanair job is the better choice for all the reasons everyone else has said. But the thing to remember is after you get your jet time at Ryanair and move on the future is not guaranteed to be any more stable. Even working for BA at LHR, the most stable job in the UK will mean a lot of time away from home. The sooner you have the "there will be times you hardly ever see me how do you feel about that" chat the better. And remember being away from where you want to be is just as hard on you even if it is to do a job you are desperate for. If you try to commute After about a year it will JUST be your job and you will likely spend a lot of time being miserable about not being at home.
Take the Ryanair job but do it with your eyes and your partners eyes fully open to what it will entail. With Ryanair there is no way of guessing what is round the corner. Assume you will get a remote Italian base on an uncommutable 5/3 contract but also assume that base might downsize or close in 6 months at which point you'll be in a Northern European base on a commutable 5/4 only to find you never fly from that base and spend your life looking for connecting flights and hotels.
On the other hand you are almost equally likely to get STN straight out of line training ahead of an FO who has been stuck in Kun for 4 years desperate for it and you will be very happy until command time.
Remember there is no fair and transparent basing system. There is no fair and transparent leave system and when you spend three months of every winter on stand by unpaid you do so at your base not where you want to be. And to top it all there is no one prepared to have an adult discussion with you on virtually any personal issue if it regards moving base.
Having said that if you settle in a base for argument let's say AGP and you like it there, as long as O'Leary doesn't decide to have a fight with the airport you could probably sit there happily for your whole time as an FO. That is the issue here. At no point as a Ryanair pilot will you be able to guarantee anything to your partner. If they can live with that then you will get a lot of good experience very quickly. However, if they have a definite plan of their own which doesn't fit in with you having to shrug your shoulders when asked where you think you'll be based in two years time there may be trouble ahead. Other opinions are available.