I think a lot depends on what the intention is combined with the budget. I am guessing from your comments that keeping the investment down is a driving force. If this is really for a college course and with no commitment for the future as yet. I think going for modest second-hand is a good idea. Remember that the lens you choose will also have an impact. If the lens or lenses you have are the same vintage as the Xti, then they are pretty old.
If you can manage it, I would go either for the T5i, or T6i over the T6 for a couple of reasons. The i-series have more robust construction, better sensors and processors, , articulating screens and generally better specs all around. They also have a touch screen interface and reasonable video specs, including earphone jack as well as mic jack.
For a general purpose lens, I would suggest looking at the EF-S 18-135 IS STM unit. It is an excellent kit lens and there are heaps of them available on the market. Not only does this have a fairly wide focal range, but it has a fast, silent 'fly by wire' motor that allows manual focus and autofocus to work concurrently - so great for both stills and video. Non STM lenses will have to be switched between manual and auto, and have noisy gears that can interfere with video.
Here Is a comparison to the different models we have discussed so far.
A good place to look is an on-line company called KEH. They have a vast array of used gear that is backed by warranty. I would also check Canon's Refurbished gear store.
cheers, TREVOR
"The Amount of Misery expands to fill the space available"
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
