Can't compare it in size and such to your FN9 as I've not tried that one yet. I do not know what the difference in between a G23 and the G17 I have tried so again no help.
I have shot the P250 in 9mm and .40. In comparing it to an SP, my preference is (and I bought) the SP. The P250 felt "plastic" to me in the context of "toy" and not "polymer framed firearm". Best I can describe it I guess. Could be the modular design of the P250 or whatever, I just didn't care for it. The SP on the other hand fit well and felt great to my hands. My first polymer that I liked enough to buy. I've tried many and in several calibers and nothing felt good enough until I came across the SP.
The size is slightly bigger, IMO in the Sp over the P250, but not by what I would consider significant by any means. The trigger in the SP is probably the most underrated trigger available in ANY handgun on the market anywhere. The one possible improvement to my P228 that I could think of would be to be able to get the trigger smoothness to feel just like that of my SP. It's a little heavier than the P250 from what I recall, but was smooth and consistant all the way through the break and reset.
Different strokes and all. Perhaps the slide weight or whaever the dimensional/weight differences are, I don't know. The grip ergos and angle are just like the other SiG's I have and that might be a little different in the P250. As soon as I get some TFO's on my SP, it'll join the rotation in CCW duty for me.
The P250 is a fine design and all, it just doesn't suit me. Many like it as do they also Glocks, USP's, XD's and so on. Just didn't thrill me is all.