Glen's work is absolutely as good as it gets in the AR world...PERIOD!
I have one of his complete builds, as well as a couple of ARs that I have put together using Seekins components (upper/lower, barrels and various other parts), and they are all top notch.
Now...John Paul is also at the top of his game in terms of work with the AR platform.
It really all comes down to what you specifically want/need, whether the JP rifles/carbines or the Seekins offerings best meet/exceed those requirements, your timeframe for needing the rifle (both Seekins and JP can have some lengthy wait times...again...depending on what your specific timeframe may be, one may work out better than the other), etc., etc. You really cannot go wrong with either company's work and I'm sure that you'll be pleased no matter which route you end up taking!