I have created a custom object for Sales Aid & Sample Requests.  The purpose of this object is to enable the user to put in a request from either the Lead or the Contact for various sales aids or samples.  Once the request is completed, it will be emailed to the appropriate fulfillment person and copied to additional internal people.

Since a Sales Aid or Sample can be requested for either a Lead or a Contact, I would like to use the same object on both of these objects (Lead/Contact).  I have a Lookup field for Contact as well as a Lookup field for Lead.  My problems start when I want to use a formula field to bring in the Lead Address or Contact Address, depending upon whether the request is generated from a Lead or a Contact.  There are a few other fields that I would also like to use a formula to populate, but of course, these field names differ between the two.  Of course, I can create a "Lead Address field" and a "Contact Address field", etc., but this looks messy so I was looking for a way to (1) identify whether the request is being generated from a Lead or Contact (2) Pass by formula the correct values and (3) still be able to pull all of the fields into an email template, so even our personnel without Salesforce.com licenses can see that the sample/sales aid was requested.  I will also need to be able to pull together a single report of all of the sales aids/samples fulfilled. 

Does anyone have any suggestions or knowledge of how to identify what object the request was generated from, and then have the remaining formula fields reference and return the correct values?

Thanks for any insight as I've been mulling on this one for some time.