Personally, I like variety, but it doesn't matter so much. I would put some thought into features you do / don't want.
For example, similar creatures tend to get similar perks and similar flaws. (I SEE CON HAS ALREADY POINTED THIS OUT BUT I WILL STILL GIVE AN EXAMPLE.) Two carnivores will tend to a meat-only diet; two strictly aquatic creatures might lend to a tekimi that could, say, breathe underwater, but he might require constant wetting or drinking lots of fluids.
As far as "better" goes though, I don't really think that either way is better or worse. Just different. :3 Two different things don't get as "strong" of traits necessarily, but you might not get what you're expecting either, since it's so spread out.
Con's example of Ime is a good one. His profile is actually approved in the guild if you wanna go take a look at it. n wn
Hm... Quick question. For the monster concept - are we allowed to use creatures that only exist in video games? There's a few creatures here and there in various games that don't seem to have a mythological backing.
Depends on what it is; you can PM con or I with specific questions about that - because that borders on cosplay but if it's a monster or myth in the game we might allow it. Better to just ask us specifics and we'll clear it on a case by case basis.