Warning long post:
The biggest pro to using the Powershot S5 is the cost for about $315 (current price just checked) you get a simple to use camera that has advanced features and incorporates a lens that has the ability to take macro, wide angle, and telephoto shots and does so very well. The drawbacks are when you incorporate so much into one lens it does not preform nearly as well as a stand alone lens would, also the image sensor is much smaller which results in more noise especially in low light and while using high ISO's.
The Canon XTI is a SLR (Single lens reflex) camera which just describes how light is transmitted through the camera to the image sensor I won't get into all of the differences here. The XTI is a great camera there really are no cons only preferences as far as performance and features go, it will do everything an amature photographer will ever want to do. The cons have more to do with cost you will pay $615 for this camera with a kit lens 18-55mm (also current pricing, deal for $500 expired) which is a decent all around portrait and indoor photography lens, and I have seen some good macro shots done with this lens. To cover all of the bases that the lens on the S5 can do and going with very low cost lenses you will end up with about $1000 in lenses in addition to the camera which is beyond most peoples needs. The pros are larger image sensor which makes it much easier to capture moving objects (insert fish) in low light situations, you have the ability to use a lens that perfectly matches the situation you are shooting in, you can shoot in RAW mode, and a few other things.
What you need to consider is how much you will be using you camera, if all you shoot is a few pictures of your tank and the occasional family gathering and family events then the S5 is for you. If you shoot pictures daily or at least several times weekly and it more of a hobby than just recording an event and cost is not a concern you should definetly look at getting the XTI.
Hope this clears things up for you a little.
Forgot to tell you where you can look at them
Walmart in my area actually carries both cameras if you would like to physically hold them, I don't think I would purchase them there unless you have money to burn, or you can try your local canon dealer just be careful of there sales pitch some will try to pressure you into the XTI or other expensive model. I recomend doing your own research and then decide.