Nick, congratulations on your job offer. I would agree, you are wise to think about how the future may pan out and what fleet/type rating offers the best long term security. I would however suggest you don't let all of these thoughts get in the way of the fact you now have a job offer, the chance to gain a type rating, experience and a salary. Both aircraft types are popular models and should offer you a future if things go wrong with the new company. Only you can decide how things affect you personally like maybe having to relocate etc. Job prospects wise, the jet would probably be the best move as many experienced pilots never get the chance to progress from prop to jet. Never heard of anyone not being able to get a job moving from jet to prop. you say the pay is the same on both fleets but traditionally jet pay is much higher than prop pay so the jet would also offer better pay in the future. I would suggest you think things over very quickly and make your decision, Once you have made it, tel the company and secure the job before it goes to someone else, then enjoy it and don't spend time thinking "did I make the right decision or not"
Good Luck!