Pretty much when you buy a set of sights (front and rear) it doesn't matter as they are matched. The dovetail and height of all XDs are the same but if you are buy a front or rear only after market sight make sure they match the height of the stock sights so they will match the ones you're not changing. I do not know why Topgun Supply and meprolightnightsights.com shows 2 different Tru-Dot sights for the XD, I suspect the only difference is the color and not what caliber gun they fit. It is interesting that these two sites are both retailers and the meprolight factor web site does not list any sight for the XD.
Sig P220, Taurus PT1911, Browning HiPower 9mm, CZ75, CZ75 P01, Walther PPS 9mm, Savage 9317 BTVS .17, Browning Buckmark, Taurus Gaucho 357 (pair), Taurus Gaucho 45LC (pair), Uberti 1873 Saddle Rifle 357, Uberti 1873 45 LC Carbine, TTN 12 Gauge Double, Ruger Bisley Vaquero 357 (pair), Springfield XD9SC, XD45 Service, XD45 Compact Tactical, XD(M) 40S&W BiTone, Taurus 617 357, Mossberg 500 Tactical