What are my choices?  Is the 20" a better choice for a 600 yard setup then the 18" barrel?  It will be wearing a suppressor.

I will probably get flamed for this this one, but I have had two Bushmaster stock chromed lined 20" barrels that will do .5 moa with my handloads using a 50 grain ballistic Silvertip bullet.  It took a while to work the loads up for the first barrel, a stock XM15 rifle.  I just put a new, off the shelf from Bushmaster, 20" barrel in my QCB upper, and using the same load for the first rifle, it was doing .5 moa and better last weekend off the bench.

You posted 600 yards, so you will certainly be using something larger than 50 grains, I am merely pointing out that I have had great results with a barrel that is much less custom than many here would say is required.

Craig