I have a treeless and love it. The one I just bought is a Rebecca Underwood Soft Rider (google rebecca underwood saddles and you will find her site). They are hand made out of Oregon and made to order so it does take a bit to get them, but definitely worth the wait. I don't think she even markets them - her sales are all by word of mouth from happy customers.
She makes 2 pad styles to go with her saddles - one for high withered horses and one for normal and mutton withered. If you aren't sure which to get, send her pictures of your horses and she can tell you. I actually have both because I my horse's backs are different enough to need different ones.
Its way more secure than my previous treeless saddle and has more "form" to it and spreads the weight much better. I have extremely nice even sweat marks on my horse. The rigging is the Y rigging of endurance, it comes with a breast collar and if you are riding steep hills she can make a crupper for you. Its the only treeless saddle I know of that the price includes the saddle, pad, cantle bag, breast colar and stirrups. No other special pad required.
As for security - my horse was a brat a couple rides this spring (not the saddles fault - he was hyped up with a group ride, without the group he does just fine) and he took off up a hill with a half rear and then ended up galloping sideways across the hill and even though I was knocked of balance - the saddle didn't budge.
The more I ride in this saddle, the more I love it.
Here is a pic of mine modeled on my haflinger. Its the McCullen style with hooded stirrups, though the stirrups are hard to see since they are turned in a bit. There are many color options you can choose from and my husband is looking to order a black on black. If you ask she will send you samples of the colors to help choose.