It may help to understand how Canon rolls out their models...
Roughly once per year, Canon will introduce a new Rebel camera. The first "T" model was the T1i. It was introduced in 2009 and in that year, it was the top EOS Rebel DSLR body form Canon.
The following year, they introduced the T2i. The resolution changed from 15.1 to 18 MP (the most major enhancement) and also ISO performance got a nice boost. It became the new top Rebel model and the T1i slipped down a notch.. but Canon still sold the T1i.
The following year, they introduced TWO models... the T3i becamse the new top model. It was very similar to the T2i except it introduced a new articulated LCD display (a feature they introduced the previous year with the 60D mid-level camera and was very popular - especially among video shooters.)
But they also introduced the Canon T3 (without the "i" suffix). The T3 became the new low-end body... it was the most basic of entry level bodies in the EOS line (and they discontinued previous lower-end entry bodies.)
The following year they intorduced the T4i... which used the same sensor as the T3i and T2i... but introduced a capacity touch-screen LCD display AND also changed the focus system so that all 9 auto-focus points are "cross type" points. Previous bodies have a 9 auto-focus points but only the center point is a "cross type" point and the rest are single-axis points. singe axis and cross-type refer to how the phase-detection focuser works... which splits light into two "phases" which will differ based on focus distance. As the phases re-converge, it means the camera is focused for that particular distance. But the phase-split has an axis... so imagine you are photographing a white picket fence and the axis is horizontal. it would be very obvious to the camera sensor when the focus is off because the upper part of the picket fence would be offset from the lower half (as though you sliced the image in two with a pair of scissors and slid part of it sideways so the edges don't line up). But if you did the same thing in the oppsoite axis, then it would be more difficult to notice that the pickets were out of focus because you have to remember that it doesn't slice the whole image... just a tiny bit of the image right at the point of the AF sensor. If the phase sensor cannot detect the mis-aligment within the tiny bit of the image that it covers, then it might be fooled into thinking it was focused when in reality it had missed focus. SO... it turns out these "cross type" auto-focus points split the light on two different (and orthogonal) axes simulatenously. Now the sensor will catch the mis-alignment regardless of the orientation of the objects in your image. Cross-type points are generally regarded as being much more accurate.
Note that each time Canon comes out with a new model, everting else slides down a notch.
Earlier this year, Canon introduced the T5i... this camera is a bit more puzzling as to why they released it because there are virtually no "obvious" distinguishing features from the T4i. Inspection of the outside of the camera body shows that they re-styled the mode-dial, but the sensor, number and type of focus points, resolution, ISO, etc. etc. are all the same as the T4i. There are some very subtle differences, but not much. Consequently, when Canon introduced the T5i, they also discontinued the T4i (normally each previous model drops down a notch in the product line-up... but not this time.)
ALSO in the same year they introduced the T5... the new lower end entry model.
At some point they did finally discontinue the T1i and T2i... and I mentioned that they discontinued the T4i as soon as they released the T5i.
This means the pecking order of EOS Rebel body cameras is (from lowest to highest):
T3 -> T5 -> T3i -> T5i
You can see that the "i" suffix is on the higher end models.
There are some other differences... for example all three cameras have a "center weighted" and "evaluative" metering mode. But the T3i and T5i also have a "spot" metering mode but the T5 doesn't have "spot" metering... it has "partial" metering. The difference is that the size of the spot is much larger. The point of spot metering is to place the metering more precisely on a subject of interest and meter for just that object (the T5 isn't able to discriminate metering so tightly).
In terms of ISO performance and image quality, you'd have a very difficult time telling the difference. The T5i goes to ISO 25600 and the T5 only goes to 12800. You'd think that means the T5i wins... but it turns out it's more of a marketing difference than a real difference. The image quality at each ISO setting are virtually identical... and noise at those max ISO's are so high that you wouldn't want to use them anyway (unless you were very desperate and don't care about the noise. E.g. you are abducted by space aliens and you simply MUST get their photograph, but their ship has awful lighting... you'll take the picture at any ISO you can get ... that sort of desperation might cause me to go to ISO 25600. So far I've never been in that situation so my MAX ISO setting has never actually been used for anything other than a test shot.)
Tim Campbell
5D III, 5D IV, 60Da
