There are 25 million Gamecubes in the wild, I don' think they'll be rare for quite some time. Used they sell for ~$30USD. I have 2 myself. One is hacked. So I know what I am talking about. The problem with hacking the Gamecube it that it requires extra hardware. I sell some of that at
www.gameparts.biz, specifically, the SD Gecko which is an SD->memory card adapter. So again, I know what I am talking about.
With the Wii, all you need is Legend of Zelda Twilight Princess and you download a save file and boom: instant hacked Wii.
The Wii's cpu is based on the 729MHz 740CL where as the GC's is based on the 487.25MHz 750GX. The Wii has much more memory to work with (88MB vs 24MB+16MB of swap) and has built-in 802.11g. Try to find a network adapter for the GC these days...
The Wii also has 2 USB 2.0 ports and a built in SD card slot.
Oh, yes, my Wii is "hacked" as well.