In the US it would be a no-brainer: Go for the ATR CA... PIC time is what you need to move on, regardless of whether it's a prop or jet. Moving to the left seat demonstrates career progression. Right seat, even in a jet is considered a lateral move when you have an upgrade opportunity.
Europe? No clue... seems to be done a lot differently over there. Maybe right seat in a 737 after right seat in a prop is viewed as progression?
As an example... Back when I flew for Atlantic Coast Airlines (United Express out of IAD), I got hired as a J41 F/O... Within 6 months I could move to CRJ F/O, but would incur a 2 year lock. J32 Captain upgrade was about a year after new hire, so I would have locked myself out of Captain upgrade. I said screw off to the CRJ F/O and held out for J32 Captain. Mind you, the CRJ was tempting..comfy, jet, F/A while the J32 was a particularly nasty kind of hell. No matter... PIC time is king!