Is there anything I will lose using Nexus?
You won't lose anything and both docks can co-exist on your system without interfering with each other, so give it a try and let us know what you think, especially if you see anything you don't like - that's how things get improved.
Nexus has more options than OD, so the UI is a bit more complicated too.
I want the one with the theme builder. Gotta have somethin' to play with.
The Theme Builder will be available for/in all the different products, i.e.; Nexus, Nexus Ultimate and Winstep Xtreme, it's not like I'm going to charge for it or whatever, lol
The only difference is that it will only show skinning settings for the dock if you are running Nexus, dock and Shelf skinning settings in Nexus Ultimate, and the whole she-bang in Winstep Xtreme. And this only because it depends on the actual applications to show the results of whatever you are working on.
Again,not much different than how it was before. To skin Winstep Xtreme you needed Winstep Xtreme installed on your system.