The artillery units have to be protected(as in, walled off so units can't get near them) which I favour infantry for... the problem is, the time and effort you spend on making protective forces for your artillery is better spend on other units.
I usually go with a couple or rocket units for backup, with sufficient air cover to keep bombers off their back, and a main force of Heavy Infantry in APCs and Tanks with the Western Gear(reduced wait time is awesome...)
Depends on the mission, though...
For those interested:
Gadget Trial OST
http://www.megaupload.com/?d=nsgsq9uq
Contains the music tracks from the game(Including full version of the OP) and a couple of Audio Character tracks with
Hideki Mihara and Aizen1 doing a talkshow skit.