I have (2) Xbox's modded, one with a Xenium and one with a Smartxx in it. They are both really nice chips, so I don't think you can go wrong with either. Review the features and functions and see what's important to you, but the most important decision you should make is where you buy it from.
There are Xenium fakes out there, so buy from a respected dealer and review that dealer's feedback in that forum.
One thing I will say, is that the Smartxx boots significantly faster for me than the Xenium. The 4 meg of the Smartxx vs. 2 meg of the Xenium may be important to some, but not to me. It all depends on what you are going to use it for. I would consider myself a mid-level user. I am sure I am not using some cool functions that are there, and will defer to others.
You can't go wrong with either if you buy it from the right dealer. Just make sure before you buy that you email them and they include a pin header.
Good Luck