What wiggywag said is true. As long as you have the base libraries installed you can run apps from either one.
We'll tell you the same thing we tell anyone who asks this question...try 'em both and just use the one you like best. Alternate days if you like 'em both. Do whatever the hack you want.
To answer your question of what is technically different, Gnome is written in C using the GTK widget library, and KDE is written in C++ using the QT widget library. Anecdotal evidence suggests that Gnome may run a little faster, but KDE has other benefits due to being so closely integrated. If you have a relatively fast computer I really don't think you will find much of a performance difference at all. I use KDE myself, it just feels right you know?
Find the one that feels right. Try Fluxbox and Enlightenment too. Linux is all about choices...and you have many.