Have a HK Mark 23 and Desert Eagle Mark VII in .41 magnum.
Desert Eagles are way too big in most people's hands. I have bigger hands than most people I've met, yet the Desert Eagle is too big for me. One might say to wear gloves to shoot the Desert Eagles. However, it can't be done. I wore gloves and I can't put my finger inside the trigger. The trigger guard of Desert Eagles are not designed for gloves use. In addition, using the gas-operated system, quite a few people have trouble with it (empty shell being stuck and failed to load the next round). Either they didn't clean the pistol well, or they use lighter loads. When shooting a Desert Eagle, since it uses the gas-operated system, you cannot use lighter loads. Some encourages using the hottest loads available. Also, same for all gas-operated guns, you need to clean it well or it will have problems cycling. Being big and heavy, one does not need to worry about the recoil of a Desert Eagle. It's significantly less than any revolvers using the same caliber. Is it fun to shoot? @#$$ yeah!
HK Mark 23's have plastic frames, for those who have concerns with plastic frames. I'm not going to go into which one is better (plastic or solid steel). It's more of a preference. You decide what you want. Mark 23's have better slide finish. This is way different than the USP's (I have not seen an HK45's or P30's in person yet, so no comment there). It is one of the best finishes I've ever seen (shiny, yet very durable). Don't forget that the military did several extensive tests on this gun and it survived. In addition, most of the HK's I've shot (P7M8, P9S, UPS, Mark 23) are designed with gloves usage in mind. When wearing a pair of gloves, one won't have problem holding the slighly bigger grips at all and shooting with gloves are not a problem.
Both are big, but Mark 23 is smaller, in both length and in the grip.
Having shot handguns for roughly 17 years, having a proper grip on a handgun, personally, is the key for accuracy, thus, Mark 23 is a better gun for me.
Both guns are cool, nice looking. But Mark 23's are easier for disassembly/assembly (the D.E. have more parts and you need to know what you're doing or else you won't be able to put them together).
Good luck. I'm sure you'll be happy with your decision. As I always recommend to people, when you're having trouble deciding which gun to go for, try to save more money and get both!