This may be a problem that's unique to me, but I've found that whenever I ride in a western saddle I get rubs/bruises on my knees. I rarely ride in a western saddle (I could count the number of times in the past 5 years on my hands), and strangely feel less stable in one compared to any English saddle. Thus I think that I end up gripping with my knees, thus resulting in the problem. If you're not used to a western saddle then you may wind up with the same issue.
If stability is what you need, then a dressage saddle with thigh blocks may be the best thing for you. The trend right now seems to be for massive external thigh blocks (not a style I'm fond of, but it's what's in) so you should be able to easily find something that works for you. I ride in a Wintec Isabell, and it has an adjustable gullet and adjustable/removable thigh blocks. If you purchase the saddle new then it comes with two sets of blocks- one big and one small. They attach with velcro under the flap, so you have a good deal of flexibility with where they are located. The big ones may not be big enough for you or provide enough structure though, so someone else may be able to recommend some big, bulky external ones that would be more suitable!