I will give you my reviews based on what I deal with in CDE's. Of the three harnesses listed and if you are planning on doing CDE's, I would recommend the Country Carriage Beta bi harness. I have seen all three of these harnesses and chose the Country Carriage Beta Bi myself. It is a very classy harness that gets good scores in turnout and sturdy enough to handle an intense marathon. The highlights of the harness is ample padding, the saddle has a tree and tons of adjustablility. The Driving Essentials harness just doesn't have the classiness, but did seem durable. The Ozark I found was more suited for carriage pleasure shows as opposed to CDE's. It didn't have much on the way of padding, no tree in the saddle (if I remember correctly) and single neck straps. Any of the three would be good for beginner/training level, but if you have any thoughts on moving up the levels I would highly recommend the Country Carriages harness and if money isn't an issue go with the Deluxe model,(which is what I have)extra padding down the sides of the saddle and fancier bridle.
Now you didn't mention the Comfy Fit Harness, but I would also recommend it. I have several Comfy Fit harnesses both pair and single. I think the BetaBi is a little nicer for turnout, but is also a bit higher in price. If you are fitting a mini/VSE I would definitely get the shaped breastcollar and always buckle in traces. The shaped breast collar allows for more freedom on the shoulders and the buckle in traces allow for more adjustability. I have found that as traces go on the smaller ponies, those closer holes on the breastcollar end allow a better fit than the slots on the rear of the traces. I think the rear slots are @ 2" apart where the holes at the breastcollar are half an inch or so. On a smaller pony, 1" can matter alot. When you are doing a marathon obstacle at a full gallop you want the harness fitted as perfect as you can.
Hope this helps some, it is just my opinion and again I do mainly CDE's (hopefully moving up to Intermediate at the end of the year) so based my harness reviews on that. I think most of the harnesses are within $100 of each other so you will have to choose what works for what you want to do. I knew that I wanted to move up the levels and wanted a harness that would be able to handle it.