I wish I knew more about donkeys; I can go on the few I've looked at, but that's not really enough to generalize.
The ones I've seen looked pretty narrow and angular in front, getting very flat in the back. That second saddle looks like it might work well for that shape of back. I think you should find somebody local who knows a lot about donkeys, though.
You can check to see if the second saddle would be too long, at least. Take a measuring tape, and find his shoulderblade--the top is the boney half-circle to the left and right of the withers. Measure from the furthest back point of that bone, to the point of the hip. For a saddle which is 21" long, that length should probably be at least 23". If the saddle is too long for his back, then your weight will rest on an unsopported areaof his pine, and it would likely bridge (fit very badly).
The first saddle--are you aware that English and Western saddles are measured differently? A 15" English saddle is much smaller than a 15" Western saddle. 15" English saddle is "small child" size. Also, I've seen the pics of that "saddle" before, and I have no confidence in it. It looks cheap and badly designed--and I am a big fan of using synthetic saddles when they work. I really think that thing is a glorified bareback pad.