Rise of Nations is like SC/BW plus some civility, and a little more complexity thrown in. As someone previously mentioned, there are many more options as far as commands for units and such go, as well as the whole research and territorial aspects. It's a great game, I just sometimes wish it would somehow slow down as you go along the ages. The speed is perfect in the beginning, but once you start hitting industrial and modern, things can get a bit hectic/confusing. Then again, I'm not the best person at micromanagement.
But yes, BW is definitely worth the purchase. I got the battlechest about two years back, and haven't ever regretted it. Also have the same for WC3, and I'll still have the occassional yearning for some starcraft.