Which one?

I am really having a hard time deciding which one. I have the Sigma 70-300 Manual & think it's great I want one with a motor tho. I haven't messed with a Tamron Are they any good?

Tamron | 70-300mm f/4-5.6 Di LD Macro Autofocus | AF017NII-700

Sigma | 70-300mm f/4-5.6 DG Macro Autofocus Lens | 5A9306 | B&H

I had an older version of the Tamron and it was a good performer.

On the plus-side, it was a very sharp lens and it was versatile.

What I didn't like about it was it only had 6 aperture blades, making the out-of-focus areas harsh. I also didn't like that the front element would rotate when it focused, meaning I'd have to adjust my polarizer if there was any change in the focus. Lastly, it wasn't very fast focusing (although that wasn't too much of an issue unless I was stuck with it shooting birds).

I haven't used the sigma, but have heard it's also decent. How it performs or whether the front element rotates, I can't say though.

That's the one thing that i heard that it was slow. I think i might go with the sigma. Anyone else?

I've got the Tamron, and like Andrew pointed out, it can be a bit slow at times. But I've found it to work just fine sometimes, too.

These were all taken with my Tamron 70-300. Angie Coker-Sells Clinic 2009 - a set on Flickr

I think i'm going for this one.
SIGMA 70-300mm F4-5.6 APO-M DG HSM FOR NIKON

Those are great Is it slow enough to miss a shot?

Sometimes, but I haven't really used any other long lens like the 70-300 before, so it could be a normal thing.