Jace: Don't compare the low mercury HPS with the HG-free HPS ones.
ALTO HPS lamps have the same characteristics as regular HPS lamps but with much lower mercury dose.
The HG-free HPS lamps is a different story. They have 300 torr xenon pressure compare to 50-80 torr of the regular and ALTO HPS lamps and a longer and thinner arctube then both lamps. Also they have an antenna for reliable ignition on standard ignitors. The higher xenon pressure in HG-free HPS lamps, like in the european Super HPS lamps with mercury, preserves the electrodes for longer while also greatly improves lumen maintenance so these lamps have 95% of their initial brightness after 24,000 hours. Therefore they have more life then standard HPS lamps. Usually they are rated for 28,000 hours and the european Philips Master SON PIA HG-Free HPS lamps have a rated life of even 32,000 to 50% failures.