These were introduced on opposite ends of the Rebel line. The Rebel line is an entry "range" of bodies but within the entry range there is a low end and a high end.
Roughly once per year, Canon introduces a new Rebel model. You can think of it as "this year's model". But they don't actually stop making "last year's model". What they typically do is increment the model number but continue to sell the older model at a reduced price. The very newest Rebel body will typically have a price point which is less than $1000 for both the body and one kit lens combined. If you are happy to buy "last year's" model then you can save a bit -- you wont have the very latest features, but let's face... "last year's" model was probably a pretty good camera too -- even if it doesn't have the latest bells and whistles.
The most recent Rebel body is the T5i (announced last spring). The year before that it was the T4i (which is nearly identical except for the mode dial) and the year before that it was the T3i. The T4i and T5i both have 9 "cross type" auto-focus points and a touch-sensitive screen. The T3i does not have a touch-sensitive screen and while it does have 9 auto-focus points, only the center point is "cross type" and the remaining 8 points are single-axis focus points. The "cross type" points are a bit more accurate and often a bit faster.
HOWEVER... at the same time, Canon usually offers an entry-level body which is more affordable than any other camera body in the Rebel line. They do not update this model each year like they do with the higher end Rebels. Prior to the T3, the low-end body was the "XS".
What's common to the entry bodies is that they lack the "i" suffix on the model name. The higher end Rebel bodies always have an "i" suffix. The T3 entry-level body came out in the very same year as the T3i high-end Rebel body.
In the year when these two cameras were introduced, the T3i was the high-end Rebel body and the T3 was the low-end Rebel body.
So likely the answer to your question is: the T3i is a significantly more feature-rich than the T3. If the T3i is within your means, then that's probably the camera you'd be happier with in the long run.
Tim Campbell
5D III, 5D IV, 60Da
