Which one of different 670's should I choose?

I'd go with Asus.

Asus or GigaByte both have good aftermarket fans.

Go for which ever both are good.I personally would go for Asus.

Asus 670 TOP - 10.0 rating (only card ever to get a 10.0)
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670...

Gigabyte Windforce SC 670 - 9.8 rating
http://www.techpowerup.com/reviews/Gigabyte/GeForce_GTX...

MSI Twin Frozr IV 670 - 9.7 rating
http://www.techpowerup.com/reviews/MSI/GTX_670_Power_Ed...

Palit Jetsteram 670 - 9.7 rating
http://www.techpowerup.com/reviews/Palit/GeForce_GTX_67...

Zotac AMP 670 - 9.6 rating
http://www.techpowerup.com/reviews/Zotac/GeForce_GTX_67...

Asus Direct CU II Top is the best  
I got the 680 one and its supberb!
If there is a Direct CU II top go for it, no matter card serie xD

But there is no TOP one, DC2 only.
Is it good enough?

Yup. Very good.

As far as I have found the main difference between those cards is their core clock frequency.
So as it goes:
Gigabyte GTX 670 OC - 1058 MHz (boosted), 980MHz (base)
Gigabyte GTX 670 WF2 - 1019 MHz (boosted), 941 MHz (base)
Asus GTX670 DC2 - 980 MHz (boosted), 915 MHz (base)

Which one - boosted or base - comes from the box?
And if that's the only thing that means then Gigabyte OC is the best card.

the speed base (ex: 941mhz) is the frequency "normal" but when the vga need more power is boostes to 1019mhz.

In speed the gigabyte Oc is more than asus dc2.

Oh non top hmm... I think i would pass in that case go for the Gigabyte Windforce SC 670 - 9.8 rating
http://www.techpowerup.com/reviews/Gigabyte/GeForce_GTX...