You can continue to use X1 after upgrading to X3, most things belonging to X3 are kept in separate release-specific folders. The Plugin Manager will change to the revamped one from X3, but X1 will work just fine with the X3 Plugin Manager. There are a couple of other things, but I don't recall what they are, and in any case, they do not adversely affect X1.
X3 has been quite stable, and has had a great deal of attention from Cakewalk, in terms of maintenance, thus far. There have been 4 maintenance updates already, and a fifth one is in the works.
X3 also bring in Melodyne, as a replacement to V-Vocal, and is quite good. There is also VST 3 support, improved video support, and the full Addictive Drums, which is quite good.
You can now download an X3 demo, and check out the base product - meaning that though the add-ons will not come with the demo, the audio engine, VST 3 support, and such will be there.
Bob Bone