Kind of an easy choice - Puljujarvi.
While Yamamoto is more offensively dynamic of the two, he pretty clearly lacks ability to impact the game overall. His size will always hold him back at the NHL level, and almost any misstep or injury that slows him down could see him out of the league in a hurry, making him a bit of a long term risk. With how much he's struggled to score over the last 2 years despite premium minutes with one of the best offensive centers in the entire NHL, he's already looking like someone who would be pushed out of the lineup in a hurry if the Oilers had more competent NHL depth at the wing position. It's probable that reality will come to fruition sooner than later with Holloway and Bourgault in the wings anyways, since they will likely start making inroads towards making the team within the next year or two as the team is forced to move to younger, cost controlled players due to their cap situation
On the other hand, while Puljujarvi will never be the offensive player he was drafted to be due to his lack of hockey sense, his ability to use his size and reach means he'll always be valuable to a hockey team. It's too bad he's tracking to be more of an Ethan Moreau(albeit, a non-physical version) rather than the first line star forward everyone was hoping for, but big defensive forwards who can chip in offensively in spurts are always useful at the NHL level. Only real question about his future with the team at this point is how much the artificial point bump he's gotten being a fixture in the top six and on the powerplay will impact his salary demands going forward