Foggy,
The best TR if money is no object is clearly for one's private 737-BBJ
Just kidding - If I were going to go down that route (I'm not), I would rent an hour in the Level D simulator for each type I was interested in together with a freelance instructor for that type (so he hasn't got a vested interest in selling me a course). That gives you a chance to see which one you like flying more and hopefully the guy showing you the ropes will have some idea what the demand is like for the type, maybe even some contacts.
I asked the same question recently to the pilot agencies who place experienced contract pilots around the world and they found it impossible to answer - varies month to month from location to location apparently.
IMHO, I reckon a Boeing rating would be a more 'fun' first TR to do from a handflying point of view but the Airbus ratings are more portable between types so may offer a larger employer catchment area...
I'd be very cautious about paying 10K+ for a turbo-prop TR in case I had to do it again when offered a jet job..
You have to really weigh up the costs here I think. Even if you have a lot of money - it'll take a long time to pay back the extra 15-20k from an FO's salary. If it means you get a job only 1 or 2 years earlier because you did a TR, it really doesn't make financial sense at the end of the day but I understand the burning desire to get on..