You're not really thinking this through. There are any number of things that can feed into "best download speeds".
The 5885 is a mobile hotspot, it's designed to be portable. It's a nifty thing but you're probably not going to want to use it at home every day.
The 315 is designed to stand in one place, and it doesn't (normally?) have a battery. It's bigger, its antennas are bigger, and it will likely give you better WiFi coverage as well.
It also does depend on which bands both of these modems cover, I'm not too clued up on that. Also depends on which suburb you live in, time of day, the weather conditions... YMMV.