Absolutely I would go with the Boss LS for $39K. The LS also has the center console gauges which I like. I had it installed in my Boss but with some different gauges and it makes a difference. Anyway, just for resale value and how exclusive the LS Boss is, it would be a no-brainer for me.
As for the 2015 with the track pack. I disagree with Ford saying that the baseline they used was the Boss and that it is faster than the Boss....etc.
I've only sat in a 2015 as a passenger on the track, but the 2015 is WAY underdamped... and the IRS suffers from wheel hop way worse than a stock Boss will. It is a lots smoother especially if you are cutting the curbs on tunrs, but once you get used to the Boss, it doesn't bother you.
I haven't followed the aftermarket for the 2015, but with some better springs and dampers and probably some tighter bushings and stronger bracing in the rear, it will likely come close to the Boss, but my friend could pull faster lap times with my Boss than he could with his new 2015.
Anyway, also remember how much beefed up the internals of the Boss are. If you do decide to track your car, you definitely want the forged internals to be able to take the high RPM for long periods of time.
Enough rambling.. just buy the Boss LS!
More power, less weight, adjustable dampers, TracKey, quad exhaust, "head-turnage." I'm keeping my '13 until the GT350 appears. Then my Boss goes to my son.