The game is gonna take up that space regardless of how you buy it, so that shouldn't matter. The trade-in option is legit, but for the most part, you already know what you're getting when buying a sports game. If you know you're not gonna put too much time into it, I'm not sure why you would buy it in the first place, but that's just me.
One could argue that it being a sports game gives you every reason to buy digital. If you're buying a sports game, I'm assuming you're gonna play it for damn near a full year, and by that time, trade-in value is low as shyt, unless these places run specials, like Best Buy did with Madden this year.
I mean, I had 2K14 physical, traded it in recently (at BB...Amazon was only offering $3) for $15. I mean, 15 dollars is 15 dollars, but in the grand scheme of things, it's most likely not make or break you in regards to getting any other games. Plus, the convenience of not having to switch discs is worth it for me.