I would go JL but neither are shabby. The JL is rated at firstname.lastname@example.org so it will clearly outpower the MTX but that is alot of power for the Atlas' they recomend 300/sub for maximum output so in my opinion both those amps are going to over power them. remember blown VC's arnt covered by warranty. I would look for something about 600 watts try a JL 500/1 if you like JL.
mike, keep in mind that you are getting two 4ohm subs, which can traditionally be wired to a 2 or 8 ohm total load to the amp. the atlas has some funky wiring options, so call chad at ascendant audio and see if that applies or if you can wire it to another load. whatever load you can wire it to, you'll want to give them 600 to 750 watts. you may want to consider getting an Avalanche 15. give him a call though, he'll help you out.
Yeah JW is right. Chad explained it to me as 1-avalanche 15 is comparable to 2 atlas 15's so I would go avalanche also unless you have tons of space and just want to show off two 15's. As far as amps look at a kicker 600.1 or JBL bp 600.1 something along those lines JL 500/1 on ebay can be had for under$300 so look around you will find something.