I usually PO and use Tokyo Otaku Mode (TOM) because they have most of who I want/need and I am a new collector as well- the numerous discounts, coupons, and free shipping for orders over $150 (which let's face it is usually equal to the price of one scale) make it a really cheap option. Their stock may be too limited for some, but hey, they hold a lot of the new releases I want, I ain't complaining. XD
Otherwise, since there are many previously released figures I want, I go to Mandarake and find them there. I've used Mandarake before and the goods are really good condition- the Japanese seem to be seriously anal about labeling stuff as 'severely' damaged, when it srrived to me there was like one scratch on the box and the figure was perfectly fine xD I suppose that's what comes from a perfectionist culture?
TL;DR PO all the way for new releases; Mandarake for old ones. PS: have yet to use AmiAmi. Now that I finally have my own debit card I'mma try it one of these days.