I have to rebuild my entire system, and I'm going to install the X1 Pro --> X2 Pro Upgrade. I was wondering if there is a list somewhere of what extras I would be missing if I go straight to X2 without first installing X1...  I know Guitar Rig is one of them.   Also, am I less likely to have issues if I install the X2 upgrade from the start, versus installing X1 first?    Thanks, Jonathan

Unless you need some function that works well in X1 but doesn't in X2, I'd skip the X1 installation.

After X2a is installed, you can always custom install just the pieces from X1 that you want to have.

id just install x2,i dont see the need to install both,i did,but only because i had x1 installed previously and then i bought x2 and installed that.
there both somewhat the same with the exception that x2 is better :)
much better in my opinion.

the only reason i can see installing both is some have said its more stable than x2(in my opinion this is not true)but if something you use in x2 is not functioning properly you can use x1 instead.

with that said,i havent used x1 in months now and x2 is rock solid.

It's X2 Producer not Professional. 

You can just install the plugins for X1 from the X1 installer after you install X2.

I have all the 64-bit plugs from 8.5/X1/X2, but only X2 installed.