Bear in mind also that the opportunity to move from FO to Captain often arises much more quickly on a turboprop fleet...perhaps quite soon after the issue of the full ATPL if you are up to the job. This might be an additional factor which you wish to consider because then you really will be in demand, and with salary to match.
I believe many mixed prop/jet companies use the position of t/prop captain as a stepping stone to jet FO, with the bonus that by then you would have invaluable multi-crew turbine P1 time in your log book for a "rainy day" as well as hands-on experience of what the captain's job involves.
Looked at on this basis it's not really a downgrade....more a chance for a quick upgrade.