I have both. They are two totally different tools (see the posts above for the differences). The main difference for me, is that the web 2.0's are much better in Rankwyz. You can easily drip out the posts, so the look completely natural. I do not use the autoscrape feature on my 1st tier. 2nd tier, 3rd tier, etc, it is great for. If you manually build web 2.0 networks, then Rankwyz will save you a lot of time. If you really want to automate it, and make a lot of accounts, be prepared to buy them or make them with another tool. I use Zennoposter to create my accounts.
I currently have over 11,000 blogs in the system, and have not lost very many of them. They keep on adding more and more features. It is the best tier 1 web 2.0 tool on the market. Nuke and UD don't compare when it comes to Web 2.0's. With Nuke, they keep adding and removing the custom web 2.0's. That is ok, if you are only going to post 1 or 2 times to them. But if you want to age your 2.0's before you post to them, then it is no good. As far as UD, it is not updated enough for the custom platforms.