Update: Almost two years later!
About 5 months into getting my mare, after I originally started this thread, I got a nice used Crates saddle that fits her perfectly
. It came with a 1 inch thick Diamond Wool felt pad (not contoured, not 100% wool, with a little bit of a cutback at the wither). This one: Diamond Wool Pads - Products - Wool Liners & Pads
I used that pad for awhile and it worked OK, but I wanted a pad that was contoured with the cut out above the wither (basically the style of the 5-Star pads). My local tack store had some 5-star pads in stock and I was going to go with one of those, based on so many people being really happy with that brand, but they didn't have any color combos that would go well color wise with my saddle. I'm kind of picky about that even though I know it's not a big deal for someone who doesn't show; but, my saddle is that typical Crates' "Chestnut" that has quite a bit of orange to it and it clashes with a lot of colors. I ended up finding a Classic Equine 100% wool felt pad, 3/4 or 1 inch thick - can't remember right now, that matches my saddle colors to a T. It's the same style as the 5-Star pads, so I was happy. Not sure if this is the exact same one, but like this Wool 1'' Felt Equibrand (Supplies Tack - Saddle Accessories - Pads)
After a year of using this pad I am very happy with it and it works great with my saddle and for my horse - never any issues with a sore back or overheating! And over this past summer we were riding a lot. 1.5 hour lessons 2 times per week and trails rides of approx. 2 hours 4 times per week.
So, now that I have some experience of my own with a few different types of pads, I definitely recommend a 100% wool felt pad from any of the better quality brands (Classic Equine, 5-Star, Diamond Wool, etc). Still haven't tried any of the different types of neoprene pads, but I'm happy with what I've got :)