captainyonder, your post is somewhat ambiguous. The inference could be that your friend wants to join Ryanair but that in the meantime he has an opportunity to pay for his own type rating and get in some J31 flying - while waiting to hear from Ryanair.
If he has a straight choice, Boeing or Jetstream, he must take the Boeing every time for all the reasons given above.
If he still has to wait to hear from Ryanair then perhaps he should go ahead in the meantime with the J31 flying. He will at least be up to speed, current and earning something.
There has to be a reasonable pay differential between that of an FO on a J31 and on a B737. If your chum pays for his Jetstream rating and only flies the machine for a few months before joining Ryanair; it will not take very long, using this differential, even after tax, to pay back himself or whoever has lent him the money for the initial conversion.
The real danger in this is that he might get himself locked in to a three month notice period with the J31 operator. This could be a serious handicap if Ryanair were looking for someone to start a course ASAP. Balanced against this is the fact that many companies require a six month probation period with a month's termination on either side.
In this seemingly very fluid aviation market where it does appear to be a pilot's market, the best advice might be to stay as unfettered as possible whilst gunning for the serious job. Flying a J31 could hardly, in anyone's book, be called a career move and yes indeed, speaking from experience, flying a Jetstream around in European winters is not really a lot of fun, although interesting and exacting from time to time. They're all so old that the cockpits smell of generations of biscuit crumbs and lukewarm coffee and the machine is an absolute pig in turbulence too, which probably explains the coffee smell!
