If you have the money at200 is a slightly better light. Both are using similar top quality chips, but AT has better quality components throughout(drivers, heatsink, and chips).
Coverage is the same for both units. A51 came out saying the xgs was for a 3x3...since have really backed off that statement because it just doesn't quite do it. The a51's(xgs) can do a 2.5x2.5 well. Same as the an AT...so it's up to you on which you feel is the better light.
at200: 120 of the top chips(nichia's...and yes they are better than cree) power by 2 meanwell(best in the business) drivers with a massive, one piece thick extruded aluminum heatsink with 2X the hight and width of a51's heat sink fins.
a51: 72 of the top chips(cree's) power by 4 chinese drivers cooled by 2 shorter finned(lower surface are=less cooling power) heatsinks.
a51's price point is pretty good and makes it hard to pass up. But when you get down to all of the specs...apache's are still the best pre-made lights you can buy. I have been running some of mine for 4 years with out a single issue. Not even a fan getting old. Speaking of fans...a51's fans are quieter if that makes a difference.
