Keep it if you can...
Do you really need to sell that frame to get a 575...? Depending on year and condition, how big of a dent is it really gonna make in the 575 price tag?
I totally agree that you need another 575, my back hurts just thinking about a hardtail, but selling a frame that they don't make anymore could be regretable.
If it's in great shape and could be described as "Vintage" one day, I'd hold on to her and buy a 575.