errr.. HP uses Canon ink head technology. So I don't think there is any difference from them both in Ink consumption. Infact, the HPs often ship their ink cartridges half full (and since its not translucent) you don't know that.
Canon has the best cost efficient ratio, with the most affordable ink prices. If you take out their ink cartridges, they are separate containers that store the independent colors. This would allow a much better usage of the ink instead of replacing a unified color cartirdge that has emptied one of the colors. Canon has a huge advantage, thanks to the independent translucent ink cartridges and mostly liquid inside with only a small portion of the tank with sponge(storing more ink).
2400 x 1200 dpi is definately higher than 2880 x 720. Just do the math
Canon : 28800000 Dots per square inch
Epson: 2073600 dots per square inch.
Another benefit that the Canon has over the Epson is Ink head. Its replaceable. If you look down this forum you'll notice that there are a few weary HP and Epson customers that cry frankitically, simply because the ink head doesn't print anymore. The problem is that the ink head isn't replaceable and requires a manufacturer warranty or repair servicing for the entire unit. Canon has a simple solution that allows you to buy the actual ink head combination, just drop in and replace. Its like replacing a new printer.
Personally I own a S520. All-purpose printer that replaced my old ink head malfunctioned Epson 740. The printer is beautiful with the quality despite not being a photo printer. On photo printer, its very good quality at decent speeds. The printer prints at 12 ppm Black and white and around 50 secs for a full photo quality picture. I would suggest going for the s750 or the s900, and skipping the s820 since theres no real benefits of it over the S750. The extra photo color cartridges really don't show much of a color advantage over the s520.