Should I look for Aluminum or plastic enclosure for UPS
A quick survey of the half dozen UPS units from 3 different brands sitting around my workroom indicate plastic as the standard UPS casing material. 

Here are some possible reasons for this choice:


Light weight, thereby lower shipping cost, and easier handling during set-up.
Safer, as any mess-ups / short circuits are unlikely to expose live power on the casing
Safer, as any battery spillage or leaks are less likely to eat through the casing and damage person and place.
Cost of manufacture: Besides the initial cost of design and production tooling, recurring and materials costs ought to be lower than for metal fabrication. This one is worth validating specific to the plastic of choice.
Less noise, as plastic tends to absorb the humming of various power elements very well, compared to metal.
Structural longevity, since metal cases tend to ding and dent more easily than plastics.


Now the flip side:


Aluminium casing will reduce radiated EMI
Better heat dissipation if the casing is suitably used for heat-sinking
Lower production cost in small scale: Off the shelf extruded aluminium and metal fittings in a wide variety of profiles and sizes would allow screwdriver technology to produce strong, professional looking casing.
Seems to be the casing of choice for at least one brand's large, high capacity UPS, so it isn't unheard of. The unit sure leaves big dents on the wall if rolled around on its casters, though.