There are other solutions to smoothing out the finish, and to making both slabs of concrete look like one piece. There are overlay products that you can use that will not only fill in the rough parts of the new slab, but will cover both and make it look like one piece. If you find a decorative concrete company, they can help you with your issues.
There are a few really good decorative concrete contractors in Northern VA. There are also some articles posted on http://www.novapros.com/blog
about other concrete issues you may be interested in.