Any that meet any of the racing sanctioning body requirements are a step up. Check craigslist, etc... as you often find bracket racers or round track racers selling their out-dated harnesses for cheap. I lucked into 2 sets for $8. One set was 6 years old and one 4 years old. Still 100% serviceable. Ignore the date codes as the actual racers have to replace them regularly but they are dandy for our purposes.
In my
Limited experience the crotch (anti-submarining) strap can be a literal pain off-roading if you have it tight enough to work. You could always leave it unconnected unless you were in a really hairy situation though.
Keep in mind that for on road use many states require whatever seat belts are in place to be used. If your jeep originally had only lap belts and you upgrade to a 4-point you could possibly be ticketed for not at least using the harness as a 3-point as most states require when present. Even upgrading to a later factory three point belt system in a jeep that only had lap belts originally would mean you have to wear the attached shoulder belt to be in compliance with any restraint system laws.
Incommando: All good points, currently NHRA has required now that we use a 6-point system with the addition of a second crotch belt. Years back we use to send the belts back in every 2 years for re-certification and the belt manufactures would just cut off the old hardware and sew on new belts with a current SFI tag. Now they will only sell you new belts at a cost of $500+ set. Point is as mentioned above there is a ton of that stuff out there 4-sale very cheap and it is a much better system than any stock belts.
Most belts have the quick release fixture attached to the crotch but in most cases you can run the 5-point fixture using just the 2-point 3" lap belt. Personally I have no problem using the 5 point system in the Jeep even running them loose as some level of protection in the event of an accident is better than nothing.