1) Upright or standard
I really don't like uprights. I find them hard to handle and super-heavy - not my first choice. While some uprights have detachable "mini-vacuums" for stairs I find the whole process of using uprights to be frustrating. My vote is for the standard hose with separate motor.
2) Bag or chamber
I really don't like changing vacuum bags. Who does is probably what you're thinking? I would recommend a chamber that can be emptied directly into the garbage. This is also environmentally-friendly and cheaper in the long run.
3) Cheap or pricey
This of course depends entirely on your budget. I suggest a mid-range vacuum simply because it should last longer than el cheapo while maintaining good suction.
So there is my formula: standard with a chamber in the mid-range for price!