Depending on the type of tank SPS, Mixed, LPS dominant etc I think the extra money is well worth it. I have never used the XM 15 K bulb but they get pretty bad reviews as far as PAR from users of this board.
I currently use the Iwasaki 15 K bulb on a M57 ballast ( which comes out at about 71 PPFD/PAR ) so they will fire fine on that ballast but have a bit less PPFD. The Iwasaki's are 86 PPFD on electronic ballasts. Compare that to the XM 15 K and they are only at 39 PPFD/PAR on a M57 ballast.
I have also used the 175 watt 10 K Reeflux bulb and liked the color. Not sure how they test out but the 250 watt 10 K Reeflux tests out at 70 PPFD/PAR on a standard M58 ballast so I can only assume the 175 watt version is much less. The Iwasaki 175 watt 15 K bulb matches the 250 watt Reeflux even on a standard ballast. IMO that is hard to beat.
I chose to stay with the M57 ballast for now as I also have a few LPS and a couple of softies. If I ever went all SPS I would upgrade to an electronic ballast to take advantage of the 86 PPFD.
The color is nice on the Iwasaki as well. Bright white. If you like a lot of blue, the Iwasaki will not give that.
I guess in long, I think the extra money is worth it for the Iwasaki.