The Canon i550 has an option to print duplex in the driver, but it is a manual option. You must flip the pages over yourself, and then print the opposite side. The HP 6122 has a built-in duplexer, so it will automatically flip and print the opposite side, but this slows print speed somewhat. The i550 has lower ink costs, slightly faster text speed, much faster photo speed, and similar print quality. But the i550 is slightly more dependant on the paper quality than the 6122, in my opinion, as certain papers look really good, but others can make colors too bland and/or text fuzzy. But if you use each manufacturer's papers, than both can produce good quality.
The HP 6122 tends to look slightly better, IMO, on typical business graphics, charts, etc. which is not a big concern for you. But the 6122 prints the better grayscale graphics. And the built-in duplexer is a big plus is you do a lot of reports, letters, etc. On the downside, if you do a lot of printing, the 6122 is more expensive to use as ink cost tends to be slightly higher than the i550. In addition, the 6122 costs nearly twice as much to purchase. If you'll be printly mostly text, then either will serve you well. The big question is whether the functionality of the built-in duplexer and slightly higher ink costs offsets the lower costs and slightly faster speed of the i550.
I use them both and like them both. I use the HP at work, but use an i550 and i850 at home. In addition, for printing schematics at work, I also use the Epson C82. The quality and ruggedness of the Epson durabrite ink is amazing. It resists water, alcohol, and even an acetone spill failed to ruin a printed electronic schematic. The paper curled after drying, but the print was fully readable and totally undisturbed. All this on plain copy paper. Amazing.
cd187