Were do I start, breifly I 'd say G40's are expensive to maintain(rebuild at 60k from new, then at 25k intervals depending on spec(Pulley size etc)) but also offer good boost from low rpm's and only really struggle if your requiring a high revving engine(though mine is good for 7.5K on 65mm pulley). A G40 is good for upto a 160 to 180 BHP engine.
Eatons are cheaper to obtain, with a second-hand usable one at about half the cost of a G-Lader service/rebuild. Eatons are better suited to high flowing heads, and oversize bore/piston set-up's. They can be a pain to setup with problematic pulley clearance issues, and air filter feed etc.
Thats few a breif conclusions from my limited experience and I'm sure others will have there own opinions. I'm a G-Lader myself and intend to stay that way cos I think there brill