Both work - a properly fitted nightguard should be comfortable and wearable regardless of which arch it is fitting oin. A lot of the choice has to do with the doctor's experience and preference. Understand that the guard will not cause you to clench the teeth any less, but it will protect the teeth it covers.
In my office, if you are a grinder (IOW, you move your jaw side-to-side, as opposed to clencher in the middle) I would fabricate a lower guard - This would be designed to separate your back teeth as you were moving your jaw around.
Clenching however, is a whole other ball game. Nightguards don't do much to alter a clenching habit and can intensify it. One design (known as NTI) helps, but doesn't cure the problem. And some neurologists are beginning to recommend Botox
injections to decrease clenching forces..
The subject of clenching/grinding and joint problems is a very tricky one. There are loads of opinions on what works, what doesn't and what is right in your situation. If I have learned anything over these many years, it's that the solution is a very individual one and not "one size fits all"
Dr. Rick Liftig