I like 2pc units better. It is easier to file or mill the bottom flush and bolt the extension on and have no gap. Plus, the checkering is cleaner on the other units VS the S&A part. The S&A is a pretty poorly cast part. I'm not aware of Ed Brown making a 1pc extended magwell mainspring housing. Do they make one now?
I think 2pc units are the best choice for non gunsmiths to install. Although, I have been wanting to try the Wilson or Nighthawk 1pc for a while now. They are really nice looking but way to expensive for my wallet.