Quite difficult to make the "upgrade" from Capt in ATR to Capt in a medium jet. There's no technical difficulty per se (some people will say the speed of events, but the operational environment is usually way less challenging), however you will find such a career switch quite difficult, most likely will need to fly the jet as FO for AT LEAST another 500h even after your PIC time in the ATR.
Thus go for the jet, just remember that promises are dreams unless they're written down. But at least if you don't get the upgrade, you will have a type and hours on jet to make easier jumping to another company.