The obvious attraction on AmieStreet is the sliding scale for music: An investment of $18.75 gets you $25 worth of purchases and 26 RECs.
If you are half decent at picking music, you usually make between $0.25 - $0.50 with a bit of effort on those RECs - call it conservatively $0.25 * 26 = $6.50 + the $25 = $31.50. Your average song, if you are an active user and can catch things early averages around $0.35 for the really good stuff (ignoring the large amounts of free to $0.10 cruft that you probably don't want in your playlist anyways) - so $31.50/$.35 = 90 Songs for $18.75 - with the sense of satisfaction that a significant portion of your money is going back to the artist who made the music. (Do kids still care about that these days - I am so out of touch. :-) )
I just picked up "Live, by Luna" - a stellar album for $3.22 - in a solid 256 KB VBR MP3 format. Not to mention I can both A) Play the music from the web when I wish to B) Redownload if I wish.
I guess the big difference between popcuts and AmieStreet is that you are limited to $0.98 * your RECs on AmieStreet, wheras it appears that PopCuts has a higher payout, but a larger payin per song - they are all $0.99 compared to the "starts at free and frequently pretty cheap" on AmieStreet.
It will be interesting to see how the balance of cheap versus possible greater profit works out for the two sites.
Anyways, Congrats to PopCuts for their Best Online Music Startup in Accelerator Competition at SXSW Interactive award!!