what type of release jaws are better for using a string loop? Also, how important is it to use a loop? thanks

A loop will save wear and tear on your string. It will also help to prevent the release from 'pinching' your knock off the the string at full draw. The loop can easily/cheaply replaced (see sticky tutorial by Greg/MO at top of this page). I also like the idea of holding and releasing the loop from directly behind the arrow instead of below or above on the string.

You will lose a fraction of an inch in draw length but not enough to change anything major.

With that said, there are a lot of succesful archers who shoot from the string (usually below the arrow nock).

I've never found a release that didn't work with a D-loop. The release is your preference. I personally use a Genesis Mag x for competition and hunting.

The only difference I've ever heard and Idon't know how true it is...... a single caliper release is better for someone who pushes the release really hard against their face at anchor point???? don't know but i don't think it matters

Do some shopping. There are plenty of releases that are designed to work with string loops. The head is shorter so the trigger is closer to the front. As such you aren't giving up so much draw length to get your anchor points right. Something like a True Ball Short&Sweet. There alots of others.

Is a string loop necessary? No, but if you are shooting a short bow with a fairly long draw length then it does help with nock pinch as mentioned. I've never bought that they are easier on the string and serving. Serving wear is a result of shooting and if a release is causing undo wear on the serving then you are shooting the wrong release. Any release that will cause wear to the serving will cause about the same wear to a loop. The result is that whichever you shoot from will periodically need to be replaced and the cost is about the same at most shops--somewhere around $6.

I'm kind of a different breed. 3 1/2 decades ago I started shooting rope releases for field and target. They carried right into hunting. That's what I got used to and I have about $800 tied up in releases so am not going to change. Point is I shoot one release all the time now and it's had the same rope on it for about 8 years now. With my shorter draw length I have no issues tuning or with nock pinch.

if you stand slightly behind someone who shoots with their release under their nock you can see the arrow has a slight bow to it. so when you shoot it, it has to straighten out after it is shot. with a d-loop or string the arrow is flat with no curve to it so it doesn't have to straighten out after you shoot.

Are you thinking of going to a string loop, or already have one? The reason I ask is that I have read that adding a loop after the fact can affect your draw length and/or anchor point etc. Not sure how important that all is, I don't use one.
I only hunt, and I shoot right off the string with a caliper type release (both jaws move), and a head that rotates 360 degrees. I also happen to use those rubber "eliminator buttons" to help with any possible noc pinch with my release.
My string/serving does fine without any excessive wear... and don't feel a loop is "necessary" per se. I guess it is all personal preference.
Take care.