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Between greninja or clawitzer I would choose greninja as he/she can learn a decent variety of attacks, has good stats in speed and special attack as well as having the ability protean which allows greninja to turn into the type of the attack been used. Also he/she is a ninja enough said :).
Go with Greninja. While Clawitzer has a much stronger Sp. Attack power, it's slow and it doesn't have the kind of defenses to back it up. Clawitzer might likely be out of the game before it can get a move in. Greninja on the other hand, while frail, can still hit hard and fast. He's great to use later in the game and sweep whatever needs cleaning up. With the Hidden Ability Protean, he's a monster.
That's just one man's opinion though, if you're still unsure of what to use, then check out the Battle Center's Simple Questions & Answers. They can help you build your competitive team to reach its full potential. If this is just a team to use in-game though, then try their thread for In-Game Team Help.