hello:
ksoap uses http to communite using xml, so it's not really a "versus" thing.
my main problem with using xml for communication in midp devices is sometimes the size of the xml parsers (nanoxml, tinyxml, etc). something to watch out for when your app may be limited to 50 kb on a device.
you might also take a look at kcommand:
http://www.developnet.co.uk/kcommand.htm