M7 or M9?

[BusMaster007]

Simply put, I just want a bayonet to install on my AR15 'because I can'.

Aside from that, which American made knife should I start looking for?
A plain old 'sticker'? or a 'tool'?

Which one makes the most sense IF I need to put one on the end of a RIFLE?
That would be a 20" AR15.

Thanks for taking the time to answer.

[backbencher]

As a fobbit, I bought an M7. I won't be clipping barbed wire, but if things go horribly awry, I might need to stab someone. The M7 looks like it will reduce blood pressure rather quickly, as well a pierce a vest better than a M9.