Most of this information is on the online retailer's website, but here goes:
Don't choose your fragrance based on EdP smaller size and EdT larger size. The difference in EdP and EdT lies in the oils used, not the size of the fragrance. The EdP usually has a higher concentration of oils and is generally more expensive. Since the concentrations of all the ingredients are different, the fragrance itself would be a different composition than an EdT. Test both the EdP and EdT and pick the one you like more. In your case, you know the one you tested and like, so stick with that one, regardless of the smaller/bigger size in Edp/Edt.
Some discount sites are pretty reputable in this community. I am not sure about the ones you are referring to, but I'm pretty sure fragrancenet has some customers on this board. Most fragrances are A LOT cheaper online simply because the retailer buys in bulk, and sells for much less than over priced department stores.
Testers are those bottles you use to test the fragrance in the department stores. They are significantly less because they don't come in a fancey box/cellophane packaging, etc. Sometimes, they will come without a cap. However, the juice inside is exactly the same as the retail sold fragrance, just much, much cheaper. If you don't care about the appearance of the bottle and would much rather have more money in your pockets, check out the testers.
Hope this helps, and welcome to Basenotes! You'll like it here.