I would not get either of those.
I don't see a need for a piston design in full-length AR.
The VTAC is set up the way Kyle Lamb wants it to. That doesn't mean that would benefit you necessarily, nor does that mean all Delta guys want it that way if Delta approval is important to you.
There have been some issues reported with current VTAC/Troy rail option btw, and Troy is releasing a new version.
My recommendation for a first AR is this, when it is in stock. http://noveskerifleworks.com/cgi-bin...since=&status=. You can get just an upper if you want a different lower setup. Add MOE furniture and you can mount light and VFG if you want to. If you decided you need a free float rail in the future, it can be added later.