1) In functionality
Symbian can probably handle a wider variety of custom tasks, but Windows Mobile has a lot of integration via companies like Motorola in warehouse and retail-type applications. If you’re going to deploy to information workers, Symbian might offer more flexibility. If they’re going to be scanning a lot of barcodes, Windows mobile might offer a more tested solution
2) in connectivity
Both should have equivalent connectivity options.
3) in compatibility
Windows Mobile offers a lot of great syncing with Microsoft products, unsurprisingly. What kinds of systems are you looking for compatibility with?
For more industrial uses, I’ve seen a lot of large Windows Mobile deployments but few Symbian ones. For more mobile-office type uses, I’ve seen more Symbian.
Any other ITKE pros, feel free to edit my answer as you see appropriate 🙂