I think it just depends... I'm not too sure if you would be able to get into a Q50 for $500/mo unless you found a good lease deal on a lower optioned model.
I would personally suggest go with the best of both... get a new left over 2013 G37! It's still new and stay on budget. They will be giving them away, pretty much, real soon. I'm sure you could probably get into a lease that would give you a lower note than if you financed a used G37. Lease the G37 for 2 years and save up some dough and get that Q50.
I'm kind of in the same boat as you. 27 and Graduated last year and just landed a job in the professional world. Not making a CRAZY salary, good to start, but my 07 Altima is worth about $9-10k and will payed off in Oct and I could save up a nice down payment. I could get in a fully loaded G37 for around $350/mo! Currently paying $480 on my Altima (made no down payment). But... No car note and an extra $500 a month sounds better at this point in time especially since Eboni (the Altima) is still holding up pretty well. Of course at 105k miles its time to start putting money into her to keep her running good.