[johnkeegan]

Depends on how you want to define "profitable."
Most profitable per man hour is design/install. But it's harder to grow beyond a certain size with design/install due to a limited number of "craftsman" and the personal, detail-oriented work those customers demand gets harder to control as you grow and lose "control."
Wereas, the maintenance end is more (though not totally) cost sensitive and impersonal, especially on the commercial level, and the sheer volume of work will get you more gross sales if you're willing to properly train an army of workers (i.e. Hispanics workers) and arm them with mowers, weedeaters and hedge trimmers. And, that way, you can get a greater profit through sheer volume.
Also, initially the maintenance end may be more profitable because creating a truly efficent, high-profit, design/build team requires a larger investment, with $40K skidsteers, excavators and larger trucks. At first it's a little tight but once the equipment is paid off and the better jobs begin rolling in, you can begin to see the long term benefit.
I don't know if it is easier or harder to attain that maximum profitability with design/install or maintenance (probably a mix). It probably depends on your personality and what you want to pursue as your "dream company."