I would say go for an offensive Hitmontop. This is because of the following reasons:
-You already have 2 Pokemon that can support and tank exceptionally well
-An offensive Hitmontop still retains the natural bulk that every Hitmontop has whiles a supporting one will notably not be hitting hard as an offensive Hitmontop
These are basically the main reasons.
Milotic and Umbreon or both incredibly annoying Pokemon to deal with as their impressive bulk is nothing to laugh about. Adding a supporting Hitmontop would practically be useless as Umbreon/Milotic can support an entire team by themselves.
Because of it's supporting job being lifted by 2 better supporters it would not really be of great benefit adding another one. This gives you the chance to have an extra attacker and to be frank, an annoying one
Shortly, I would say, an offensive Hitmontop would be of greater use on your team :)
