You can find a variety of articles that contrast the two levels of processors.
http://www.velocitymicro.com/blog/xeon-vs-i7i5-whats-difference/
For purposes using a Mac, then it really is decided by which Mac you get. The iMac, which only has i5 or i7, no Xeon; or the MacPro, which is ONLY Xeon.
So, if you want an iMac, you get an i5 or i7
If you need the extra video capabilities and other external versatility of the MacPro, then you will have a Xeon.
Which is better? Depends on what YOU need.
In the past, you could put multiple drives in the MacPro, along with great gobs of RAM memory.
Now that the iMac can have up to 64GB, and the MacPro only has external connections for extra storage, those are not so much a factor now.
So, the question seems simple enough, but the answer is not.
What do you want a computer to do, and which one suits your needs better?
You may spend a LOT more on the MacPro, which might be your primary decision maker.