A globeservice will give you the best performance, but it does require you to build a full cache before you can use the service.
DEM data published as an image service can also be used as elevation. In this mode the elevation is genereated on demand, and you may see surface seams and spikes, as the data is pulled in and rendered. This data is not cached, so it will do this every time you use the service.
Which way is best, really depends on the use case. For an elevation "basemap" used by many users a fully cached globeservice is probably worth the investment. You can also use the image service elevation in addition to the globeservice.. the elevation layer on "top" is the one that gets used for display.
Mark