IMO (and experience) a huge difference. The eheim is one of the most reliant filters on the market. They go strong for an easy 20 years. Versus one that leaks all the time, makes more noice, harder to get spare parts etc. Pick one
Price of the eheim is a little higher (at least in Europe) but compared to quality, well worth the extra money. If they advice the Tetra, they either need to get rid of them or make more money out of them
Not sure how available JBL is in the US, but they make some great filters as well. I think the mechanics are equally good as Eheim, the plastic feels a little cheaper but 100 times better compared to tetra and about the price of a tetra
Eheim 2217 all the way. There probably isn't a better canister filter brand out there you can buy, they also create a better flow in the aquarium than most. Get one at bigAls.com. they usually have the best prices.