While Slowbro is obviously good to keep momentum with Regenerator, i think its usefulness for the team is being overrated. You guys need to remember why Slowbro is not OU in the first place. This is because even though Slowbro is a good physical wall that can keep up the pace due to Regenerator, it hardly does anything special in terms of team support, which means that it can be easily shut down. For example Slowbro can do nothing against defensive Starmie and any kind of Celebi, meaning that the opponent will get easy opportunities to use Rapid Spin or do whatever it wants with Celebi every time Slowbro comes in. Not only this, but in later stages, where the opposing team can design and choose Pokemon able to sweep through the opposing team easily, Slowbro will be one of the easiest Pokemon in our team for the opponent to setup against, as many Pokemon can take advantage of Slowbro and there is nothing that Slowbro can do. SubCM Keldeo, SubCM Jirachi, ChestoRest Volcarona, SubDD Gyarados, SubHoneClaws Kyurem-B, SubCM Latias, and many other dangerous threats can all be potential problems for team 1 and Slowbro can do nothing to stop them from setting up.
Now let's go to Hippowdon. Hippowdon sets up sand and SR, both huge team support tools. By picking Stealth Rock early in the game we will be able to see the other team's response to it (Xatu, rapid spinner) and act accordingly (Weavile, Gothitelle, Scarf Tyranitar), meaning that it will be easier for us to keep Stealth Rock up, helping Landorus get past Zapdos easier. Also as i mentioned again, sand allows Landorus to get past Zapdos much easier, negating its lefties recovery, and building damage alongside SR nicely. Furthermore, Hippowdon has Whirwind, another wonderful team support tool, meaning that no Pokemon can set-up on Hippowdon if Hippo choses to stay in, which will be very important when the final sweeper of team 2 gets picked. Also, due to Whirlwind and Stealth Rock, Hippowdon is much harder to make a liability than Slowbro. Unless the opponent brings in a Rapid Spinner, Stealth Rock will always be useful, and even if they do bring in a spinner, they will need to spend one turn to get rid of SR, thus giving to one of our Pokemon a free turn to switch-in. I know that there is Xatu, which admittedly is a problem, but thankfully there are plenty of good Pokemon that can take care of Xatu (either trappers such as Tyranitar and Weavile or dangerous offensive Pokemon that setup on it, such as SubSalac Terrakion, SubDD Gyarados, etc) and Xatu is a very mediocre Pokemon in general.
So all in all, i feel that Hippowdon is a much better pick due to the sheer support options that it brings to team 1, as well as ability to prevent setup from anything it wants as long as it is willing to sacrifice himself, which will help in later stages of the project.