I just bought the EM500 and shortly I will upgrade the internal ram to 32 megs (www.palmpilotupgrade.com) Emulation wise I have found this machine to be unsatisfing. I can play mameboy (pacman/ms. pacman with sound and DK/DKjr without sound and get good framerates. The other emulators I have tried have been too slow. The ipaqs speed might be nice but the button issue ruins it. The EM500 can be overclocked, but I havent heard many definitive outcomes of how succesful it is.
I went with the EM500 because I like the layout of the buttons and its size (the other casios are too big). With the extra 32 megs internal and overclocking (if stable) I think would be the best machine.