If I was you i'd go with the Britannia job...
Do you really think you'll be next in line...
I don't know what that base is like but if its anything like manchester you'll be messed around.
Sorry to give negative comments about MYT.... I loved it when i worked for them, but then realised there's so many better airlines to work for.
Good Luck
Whatever you decide will be the right decision for you.