Before you start to consider configuration and design options, it's obviously very important that you make sure the harness you get complies with the safety standards for the sanctioning body you plan to race with. While SFI and FIA standards are widely accepted

Keep in mind that the certification level is also important. Tags stating the certification level are usually affixed to the harnesses themselves. The current SFI specification standard for racing harnesses is 16.1 and the FIA's current standard is 8853/98. Some harnesses may exceed these standards. the higher the certification, the more future-proof your harnesses will be.

Whether to opt for a 5-point or 6-point harness design is somewhat subjective. The 5-point design utilizes a single "sub" belt (the belt which connects more or less vertically from your groin area), while a 6-point harness uses two sub belts that meet with the camlock (where all the connectors meet) in more of a diagonal pattern. Most manufacturers are starting to move toward a 6-point design, as many racers prefer the 6-point configuration in terms of comfort and the added feeling of locked down stability that this design offers.

The most common options for shoulder belt width are 2", 3", or 3/2". 2-inch width shoulder belts ensures a comfortable fit for smaller drivers and also when using a head and neck restraint system, while 3-inch belts add a bit of bulk at the sacrifice of some comfort while wearing a head and neck restraint in order to keep the belt comfortable for use when you're not using a head and neck restraint device.
3/2" style shoulder belts seek to resolve this possible dilemma by narrowing to 2" where the straps lay over the head and neck restraint, but it is important to note that this design is not made to be used without a head and neck restraint.
Before ordering any harness, check to make sure your head and neck restraint device will accept the belt width you plan to order.
Since there are no external devices they need to route through, lap belt width is generally a decision based on the size of the drivers who will be using the car. 2-inch width lap belts are typically easier to adjust and less bulky, while 3-inch lap belts will provide more support for larger drivers at the cost of a bit of comfort.