Although jet time is always demanding and valuable. Just because you have a prop background will not eliminate you from getting a job with the legacy or any major. I have a very good friend with Delta who is a check airmen on the 767 and he has been with Delta for 25 plus yrs and is coming close to retirement. The number one advise he has given me is to get the PIC turbine time whether its on a jet or prop will not matter. He said when you have a 5000/6000 hr candidate with no 121 PIC vs a 3000-4000 hr Prop guy with 1000plus PIC, more than likely he will get the offer unless other credentials of the jet guy are impressive. I have a very good friend who just got on with United and his background was with 2 regionals flying the Dash 8 and Beech 1900D and he just finished his IOE with United on 737. So i dont think the type matters its the quality of time and experience that matters.