This is only theoretical because I haven't done a side by side but, having the extra blue in your spectrum should improve the quality of the bud. LED light designers use a ratio of 9 red, 2 white and 1 blue. They have done extensive side by sides. Plain HPS does have some blue in it's spectrum, but it is certainly not at that ratio.
Speaking of LED, you could probably get away with a Hydro Grow 126 watter ($425) in that size grow space. From what I understand, 30 watts per square foot is where you want to be with a properly designed LED, although that does depend on how hard they are being run and what bin they are, so judging LED coverage by wattage can be tricky.
$425 is probably more than you had in mind for a light, but HPS bulbs suffer lumen depreciation much faster than LED. If you consider electric savings, assume a 126 watt LED = same yield as a 250w HPS. 126 watt LED actual draw ~150 watts. 250 watt (magnetic ballast) HPS actual draw ~300 watts. Assuming 15 cents per KWH, HPS cost $2000 to run for 10 years. LED cost $1000 to run for 10 years.
Something to consider anyway. Good luck with your build!