I've used both...
(ahem...my two cents worth...)
Short version: get zaino, it kicks meguiar's in the rear
Longer version: I've used both products/systems, and here's my take on the whole deal. The Meguiar's system is good. It works well and gives good results, but it is limited in a few ways. First, it takes a lot of work to apply and remove those products. We're talking serious tennis elbow: buffing, reapplying, etc. It's just way too much work. In the prep (i.e. polish) department it's hard to beat meguiars. It'll take off all those swirl marks. But you can't beat the protection and shine of zaino. I live in mid florida, and the constant sun, acid rain, dirty college parking lots, etc. haven't been able to detract from my Zaino shine. The environment down here eats through normal wax in about two months, but three coats of zaino have lasted me over a year and a half in the past. If your car is older or in need of polishing use the meguiars polish to get the paint looking pretty. if you don't really need to polish use the zaino clay system to clean the paint and then zaino it. if you're somwhere inbetween clay it and then use the z-5 polish. I recommend the meguiar's prodcuts for heavy polishing since z-5 is pretty light and nonabrasive as polishes go. but if you decide to use a meguiar's polish before using zaino make sure you dawn wash the car to get rid of those so zaino has a nice all paint surface to bond to. In the end you'll be so happy w/ zaino you'll never think about using anything ever again. I know it seems kinda shady with the whole mail-order business and all, but you really can't beat it.