For USP models I would say the 45 compact is probably the most popular. Main reason is because of all the people who carry them. The USP in 45 does seem to be the most popular fullsize gun as well. I am thinking because in a full size gun in 45acp there arent a lot of high cap choices until recently. Even with those new choices the HK is the nicest to shoot. All of them will hold their value well, buy what you want and you will be happier. I will say the 9mm seems to be back on the rise with the increase cost of ammo. I for one am buying and shooting guns in 9mm a lot more then 40 or 45 simply because of the cost and putting a hole in paper or to compete with the 9mm is not at a disadvantage at all.