In my opinion, the benefit of a mod is it will give you exactly what you want, once you figure out what that is.
Do you want longer battery life? Go with a bigger battery, or more batteries, or more, bigger batteries. My current mod has 2 18650s and I have to admit I've been thinking about 2 26650s somewhere down the road.
Do you want more vapor? Get a higher voltage mod or a variable voltage mod. My 18650s are driving a 5V regulator.
Do you just want something that looks really cool? There are many options and I don't want to offend anyone by leaving out their favorite but the Empire mod, anything GG, Reos, Provari (black chrome, drool) and the Precise line are some that appeal to me.
Maybe you don't have a lot of money to spend. Hello BoxMod. Even better, get into modding and make your own. I've gotten a lot of questions/comments about my Altoids tin.
Maybe you want something that not only provides your perfect vape, but will also mow the lawn, take out the trash and make you a sammich. I think that's something you'll have to make yourself. If you do, please consider becoming a supplier here. I think you'd do well.
To answer one of your questions, a true 3.7V mod will let you use lower resistance attys/cartos that I would avoid on an eGo. I won't use anything below 2 ohms on an eGo for fear of overheating the electronics in them and ruining the battery; some do and get away with it while others find they need to buy new batteries. Actually, you can repair them if you're good at that sort of thing but I'm not sure if it's worth the trouble for me.