Gonna go out on a limb and say something I haven’t seen posted yet…how long a saddle can your horses back accommodate healthily?
My horse has a short weight bearing section of his back even at 16.3h, I am 5’11". Technically I can fit up to an 18" on him but they tend to make him a bit sore in his lower back even though it technically fits on paper. We compromise and I ride in a 17.5" Stubben with a freakishly forward flap (pray for me when I try to buy a shaped hunter pad). I am balanced and my horse is happy. Yes, I probably need a bigger saddle technically but this combo is working really well. The plethora of 18" saddles I have tried and the 18.5" I have tried were all a big no from my horse and that’s ok.
Pretty sure, but correct me if I am wrong, that those saddles are Stubben Phoenix Elite’s which I love. I ride in a Portos Elite but the Phoenix were my #2 choice. Tend to have a flatter seat, narrower twist from my experience a great spot for riding. Stubbens also place the rider close to the horses center of gravity, so closer to the pommel than the back seat, never had an issue with it chafing or being uncomfy. I had a nightmare of a time with CWD and Voltaire though on this but that’s irrelevant to this thread.
TBH neither saddle looks balanced on the horse which I get because it’s just demos. I think a lot of the issues with the 17" is because it looks pommel high which you are compensating for by leaning forward and enhancing the “'too small saddle” look in the pic. Likewise, the 18" looks like it might be cantle high just standing still which we obviously know is no way to check a fitting saddle, it needs to be assessed in motion for horse and rider but I digress. Flap on the second one is good for your leg (are you in jumping or flatting stirrups?) I would say but I think the balance of the seats are making things look different from how they feel if they were balanced particularly in the 17".
I also know from my Stubben fitting, some saddle demos in the 17" range had weirdly small flaps, narrow and straighter and short also enhancing the too small look. TBH the seats don’t look all the different to me, a bit more room up front in the 18" but even though the back bit on the 17" is cut off but there looks to be similar length behind. I think the 18" looks better because the flap is better. I once sat in a 19" Tad Coffin with like mini flaps and it looked too small on me because of the lack of flap even though I was swimming in the seat. So a lot of people are saying the 18", I would be inclined to go 17.5" max and just find a better fitting flap for your leg. I think a 17", considering your size (cause you aren’t almost 6ft like me) would be fine with an appropriate flap, that one is just not forward enough, if it were the comments would be different. How the seat feels is very important. I know people are saying “it’s not just what it feels like it’s what is the correct size either way”. That’s true yes, but in my case as explained above, it was very much not the best to go with what technically was correct. Too small a saddle is just as hard to balance in as too large a saddle the balances are all off either way. If it were me, I would purchase neither of those saddles, and find a 17-17.5" with a more forward flap or order new if you were planning on it.
Also, FYI Stubben has great after market modifications available. So if you find the tree and seat you like but need new flaps in a new size they are fabulous. I have worked with them before on making my tree a size narrower and changing the leather accents because I bought mine second hand and hated the accent leather color. They are great so there is that option as well. Kind of enjoying how involved the replies are getting, props to you if you read through them all