You need to put those bikes next to each other and then stand back to the side and look at them. There is quite a bit of difference in the Sirrus and the Dual Sport for the position given in the same size. If you have your FX with you, put that with them two. Then you'll know how much difference from that if you compare your contact points on the bikes (butt & hands and then BB center).
Sirrus in the same size should be the higher more upright position. But coming from your FX which is lower and more aggressive than the other two, then you may prefer the more in between the two position the Dual Sport gives.
You can try a size down or up of each. Larger size will give you less saddle to bar drop, but might stretch you out a little more. Smaller size will do the opposite. Be aware of things that change on the bike as size changes. Such as bar width and crank length