is the trendy choice, gives instructions to manually create your pages. requires flash 9
is probably the cheapest, buy the basic package get the flash viewer included. requires flash 9
is a bit pricier but includes quicktime creation and works with flash 8 which makes it more compatible but a little shakier.
All in all most people go with flashpanoramas.com but I rather like the look at immervision's which strikes me as a little smoother scrolling. They both have a quite nice update your flash plugin system for customers who are not up to date.