I drive my car a lot. How do I know which of these seats is the most comfortable? Will all these seats be more comfortable than my factory seats?
In general, most racing seats are more firm that factory seats, since most racing seats are made with performance and lateral support in mind. So that means that while they won't necessarily be stiff, they're certainly not designed specifically to be cushy for long road trips either. That said, there are things you can do to maximize comfort with aftermarket seats. A lot of seat comfort has to do with proper adjustment, and this applies to factory seats as well. Always try different positions, along with various steering wheel adjustments, to find out which combination works best for your body type and height. Your knees should be slightly bent when touching the floor, without having to stretch to reach the pedals. The seat back should put your shoulders behind your hips, ideally so that your upper torso is at a 100-110 degree angle to your thighs. If your seat has other fine adjustments such as height or lumbar, play around with them until your seating position feels just right.

Can I use my factory seat belts with aftermarket seats?
In most cases, it is entirely possible to maintain your factory seat belts with aftermarket seats. On some vehicles, however, the seat belt buckle is integrated into the seat itself, so it may be necessary in those cases to use aftermarket harnesses that have been properly installed according to the harness manufacturer's instructions.