CharlieMac is right on when he says you need lots of hard drive space. Depending on how you encode, it's not unusual for a one hour program to take three to four hundred MB of space.
Some points:
1. Make sure you buy one with digital TV support. Sounds silly, but there are still tuners out there (they're cheap!) that are analog only.
2. Consider whether you want cable TV support.
3. Consider spending more money for quieter drives and fans. If you're using this as part of a PVR system, than spending a little more money up front will save you the headache of the high whine of the system fans.
4. Consider your video out plans. How are you going to present the video you record?
5. Consider the software management. Actually, most tuners are relatively comparable. They key distinction is the software you use to manage the hardware. What are you going to use to manage all your files and interface with your tuner?