which is better editor for C++ programming XEmacs or Emacs?
Emacs has long had GUI support on all platforms.
Other than just preference for look and feel, the only think I could think of is that I've heard there are some elisp compatibility issues between the two.
Edit: EmacsWiki has a EmacsAndXEmacs comparison article.
Wikipedia description of differences between GNU Emacs and XEmacs.
In my opinion, XEmacs development started to stagnate several years ago, after release of GNU Emacs 21. Most elisp packages are written for GNU Emacs nowadays and they aren't guaranteed to work in the XEmacs.
IMHO there isn't that much to chose between them for C++ development as the basic tools work for both. They're probably working slightly better in GNU Emacs these days as I get the impression that fewer people are targeting both these days. Both have a GUI, but in my experience you're better off switching off parts of it after a short while.
I've been a long-time XEmacs user until about two years ago when it became clear that compared to GNU Emacs development, XEmacs development seems to be lagging behind considerably- used to be the other way around. I managed to switch without too much effort and have been using various GNU Emacsen ever since. Also, you'll find that a lot of the recent ports of Emacs to other platforms like Carbon Emacs, Aquamacs or EmacsW32 are all based on GNU Emacs.
Steve Yegge makes some great points on why you should use GNU Emacs. Along with having a great article on using Emacs effectively. These days, I'd stay away from XEmacs unless you have a specific reason to use it.
Currently is better to use GNU Emacs, as it has more features now, comparing with XEmacs. And Cedet currently better works on GNU Emacs
Xemacs is not under active development, so emacs is the only choice of emacs implementations.