To max out a roserade or mamoswine?

Mamoswine will be far more useful, as the optimal counter to a few legendaries and with flying dragons (eventually Garchomp too) very popular as gym defenders.

There is little need for Poison attackers, as Metagross is far better at dealng with fairies and the elite Fire-types are far better at dealing with Grass-types (many have a secondary typing that negates their Poison weakness).

Likewise, Grass attackers are generally mediocre. If you need a Rock/Ground counter, use the top Water-types instead; if you need a Water counter, Raikou is bulky (and many Water-types have Ice-type moves).

Mamoswine. It's useful straight on for gyms, and I have too often lacked in the past the Ice power needed, covering it with sub-par options like Mew/Mewtwo and even Lapras because I was always reluctant to invest on Jynx. Weavile gave me a reason to power up an Ice mon, but Mamoswine is so much better than anything else.

Do you have a use for Roserade at the moment? Me not, but it's of course a great option if you really need Grass or Poison horsepower.