The saddle and breeching are the same on both harnesses.
With a breast collar harness you have the breast collar, the neck strap, and the traces,(sometimes the traces are not detachable from the breast collar).
With hame-style you have hames, collar, and traces, traces are always detachable from the hames.
You cannot use the two styles together, it is either or.
I still suggest the breast collar harness. If your horse gains or looses 100 pounds it will make your collar not fit. I know people that have several different sizes of collar to use depending on the time of the year and how fat their horse is.
I know a woman that bought a Freedman's collar and hames....$5000, it is too big for her horse now.
I have an empathy collar, gives a little more shoulder movement.
Have you looked at Zilco harness, I just got one and I really like it.
I got a combination of the classic and the elite with the empathy collar.
It is nice because you can buy individual pieces from different harness to create a custom harness. Carriage Driving Harnesses
Classic Carriage Driving Harnesses
Elite Carriage Driving Essentials