Are these two routes the only options? PHL and LAS aren't exactly near neighbours.

If the ultimate route is LAS-XXX-MAN I'd still choose the knackered 747, though might prefer LAS-LAX-LHR-MAN (yes, two stops!) because of the better BA product and maybe the option for AA as well.

Simply, a long economy feed into the PHL-MAN involves a lunchtime cross-country flight, followed by a relatively early departure (9 p.m. Eastern, 6 p.m. LAS) time, landing in the early morning and being at least 12 hours from a bed.

LAS-LHR-MAN allows you to try to get some sleep as soon as the plane takes off, you get your four or five hours, then get to MAN, pootle around for a few hours in the early evening with dinner and checking in to your hotel, and hopefully get to sleep at a relatively sensible time. Different people deal with jetlag early, but for me, west to east is always a nightmare but late departure and late landing work best.