I personally prefer omitting a Grass-type from my team simply because most Grass-types are so inadequate; that being said, I don't know your team at all so I'll just assume that you want a Grass-type and I'll deliver you the best alternative!
I'd say between Amoonguss and Roserade, Roserade is the better alternative. Amoonguss' fantastic HP stat fails to overpower Roserade's awesome Sp. Defense, Sp. Attack and Speed stats. Roserade can also evolve earlier than Amoonguss, who evolves at Level 39. Nothing else really sets these Pokemon apart though - same typing, similar abilities and a fairly bland movepool. But do I have the Grass-type for you!
Even though it looks like a weird Pokemon, Ferrothorn has many great characteristics that I think outdo the other alternatives. Ferrothorn's magical Grass/Steel typing resists 10 types - and that's not counting its immunity to Poison, a crippling weakness it would usually have as a Grass-type. Furthermore, Ferrothorn has fantastic Defense and Sp. Defense stats and a sufficient Attack stat to support its incredibly diverse movepool. You can also catch Ferroseed near the same area you encounter your first Foongus (above Route 6 in Chargestone Cave). Despite all this, Ferrothorn does have an abysmal Speed stat and a 4x weakness to Fire-type moves. You'll also have to deal with its less-than-amazing pre-evolution Ferroseed until Level 40, although if you were considering Amoonguss, that might not bother you.
Sawsbuck is a nice alternative too. Not only can you choose to make it look like a walking palm tree, you can make it learn Double Kick and Jump Kick to support its decent Attack stat. Deerling is also a bit better to cope with than Ferroseed too, and evolves earlier at Level 34.
Remember, it's your game so play with whichever Pokemon you think you will enjoy playing with.
Hope I helped. :)
Source: Mentioned Pokemon's respective Serebii pages