With most manufacturers the product with the higher number is better, but may include features you don't need. This includes HP.
The 800 series are PhotoRet II and will print at 600x1200. The 900 series are PhotoRet III and print at 1200x2400. Those two printers are about the same speed. So it comes down to resolution. Will you print pictures? If yes then get a 900 series. If not an 800 series should do you just fine.
- JW