The massive difference, and main point of these managers is to let you know what's installed and what needs updating - Steinberg, despite the additional apps required doesn't even show what you have installed. I find that ridiculous personally. I'm fine, as used Steinberg for years - but anyone new to the platform have a struggle on their hands which isn't helped when you have a number of different Groove Agent & HALion's to install. It's those users which I feel for.
The download manager should simply show you which products you own, which are installed and which have updates. Otherwise what's the point? That's what Native Access does, so is much more justified. (imo)
NI's problem is that they don't want you to keep the installer files, if only they provided that option for people on slow connections.