The only difference, is that the majority of this series of lamps require a closed fixture with a front cover of hard glass, whereas the vast majority of SON lamps can be operated in open fixtures.  There are however versions of this lamp with the -P suffix which are protected by a Teflon coating on the outer envelope, making the lamps suitable for use in open fixtures. 

As with SON lamps, these are available in a huge range of wattages, starting at 250W (which is admittedly rather high compared to the smaller SON lamps), right through to a maximum 2000W - giving out a rather crazy figure of 200'000 Lumens.  Not one you'd want to put in your living room that, not unless you want to leave your shadow burned into the wall when you leave anyway!

These metal halide lamps are slightly less efficient than their SON counterparts, the 400W lamp here achieving a respectable 89.7Lm/W, whereas its SON equivalent comes in at 120Lm/W.  The figure is still highly respectable though, and the approximate 25% drop in efficacy when fitting this HPI lamp rather than a 400W SON lamp is likely seen as a worthwhile sacrifice in return for getting clean white light with good colour rendering abilities in a lot of cases.

NOTE: I do not presently have gear on which to operate this lamp.  Operational photographs and a spectral analysis image will be added once I acquire such equipment.