Not a hard decision to make really
You're on rubbish wages as an FO on either type and they're both stepping stone jobs to better ones right?
Do 18 months at Ryan you'll have 1500 glass cockpit jet and you'll be in the ballpark for your local highly paid national airline / Emirates / Cathay or whatever top shelf job you fancy.
Do 18 months in a J31 and you'll have, ummm, a few hundred hours on an clapped out obsolete little 1960s turboprop, and you will probably still have to go do a couple of years at a lo-co jet operator before you can get a real job.
Or, after 3 years you could get lucky and have your J31 command, earning the big 3-4,000 euro for the rest of forever, hand flying around Europe at FL200 and 230 kt right through the middle of all those winter CBs. Meanwhile your mates who went to Ryan also got their command after 3 years, they are sitting above it all at FL400 with an autopilot in, watching the plane fly itself and earning 8-10,000 euro a month in the LHS
Nobody takes the J31 seriously any more, even if you've got 5,000 hours command on it, you'll still be lucky to get an interview for a jet RHS.