I had a same kind of choice to make some time ago, when I was an experienced ATR FO. In my company we had both ATRs and jets, and my options were to either become an ATR commander or to fly a jet as an FO. Of course I would be able to apply for ATR LHS later even if I chose jets, which made my decision easier. I chose the jet and I do not regret it, even though I did find the ATR more enjoyable to fly. In my view, flying a jet has given me a lot of new and valuable professional experience, from which I can draw from when (and if) I later became an ATR commander. When I was an ATR FO, I noted that ATR captains with only ATR experience often lacked "airmanship" and the ability to see the "big picture", but captains with jet experience (even with less TT) usually had more confidence and better airmanship in general. If I were be in the same position as the thread's starter, I would of course take the "free" command upgrade, especially if the terms and conditions are good, but in my honest opinion, having both turboprop and jet experience makes you a better pilot and a captain.