It depends on what phone are you using right now.
If you're using a business-type phone like blackberry, Nokia E72, i guess it is better for you to choose iPod Touch as they are completely 2 different things.

In the end you will have entertainment, music on ipod touch,
and quick texting (with qwerty keyboard), email, instant messengers on the business phone.

Furthermore if you sign up for data plan, you can actually turn ur business phone into a wireless access point anytime, and let your ipod touch connect to it. With that way you can actually browse net with your ipod touch anytime. (downside is, it drains your business phone's battery very quickly. And costly if you're not using unlimited data plan)