First off, both fixtures are Proven and either one would work.
Hands down the ATI is a much better fixture then a standard TEK.
(This could easily be your opinion only)
The ATI:
The ballasts, the reflectors, & [the addition] of Fans - Are of much better quality.
With the design of their aluminum housing/heatsink being slightly thicker it does hold in more heat, thus more of a need for a fan to cool it down. Equipment aside, I am sure that a german reflector by any other name would still be a german reflector.
The TEKs
Cheaper, Bulbs will loose their output faster [no fans] Runs hotter and the ballasts will produce less par then that of an ATI,
(This I feel would be the poster's opinion)
I have yet to even feel the aluminum housing to be anything but warm. The wider the heatsink the more it is allowed to cool by air that passes over, since heat is spread over a wider cooling area. The shorter the area the less cooling area is used. Tek lights also use german reflectors, solid state ballasts which do not emit heat. So any heat would only be transmitted through the bulbs. While a fan may also assist with the release of heat through the bulbs, you can step up to a Tek light elite which does have a fan.
As far as 4 VS 6
The 6 bulb will allow more color combinations, but more heat & electric.
Either one would be fine for a 40 breeder.
(I agree completely on this)