They can both offer similar things, but Android is more modern and user friendly. Symbian is only used by Nokia and they have decided to drop it now in favour of Windows Phone 7 (from Microsoft). Android will still keep going for a long time yet, so it will continue growing and getting better, while Symbian is a bit dead in the water. In terms of features they are probably able to offer pretty much the same things, although Android is a lot more customisable. It is also easier to use than Symbian. Symbian would have been a good choice a few years back but there are newer and better things now - Android being one of them.
Android is fairly new and has a smaller market share than the older Symbian
Android is designed around touch capable devices while Symbian is designed around keypad devices
Symbian is somewhat outdated while Android is constantly updated
Android is used by many phone manufacturers while Symbian is almost exclusive to Nokia