It's both.
You have to install a system and institute it as a program if you hope to sustain a team. If you don't have a system that dictates the types of players you need, your chances of hitting on draft picks drops, because you expose yourself to more variables when you're trying to find a player that will fit any scheme.
By the same token, you consistently have to adjust your system based on league trends. You also have to adjust the system based on current strengths of personnel. You may have the types of players that you like for your system, but this may be a year that you have a really good secondary, for instance, and want to showcase it. Sometimes, maybe the type of player you want isn't there in the draft, but you've got a super-talented, can't-miss prospect falling into your lap, and you adjust to take advantage of their talents, but that's more of an exception case. I don't think anyone in the league can just consistently be dealt a new roster of players every year and build a whole system around them. It's about incremental adjustments when needed.