They are both of exceptionally high quality and backed by folks who you can count on if, God forbid, something goes awry and you need CS in a rush.
That said, having used both, I prefer the Seekins thanks to the Ambi-bolt release, overall ergos/fit/finish, elimination of all the @#$%ing roll pins and relocation/captivation of the takedown pin spring/detent, and the appearance of the Seekins lower which just seems "cleaner" to me than the Stiller lowers. I will say that I think that the Stiller lowers are likely a bit more "bombproof" in terms of their overall design/construction/materials, but that's from purely my observation...I don't think that in 99.9% of the uses where either lower/rifle/carbine will be fielded will make an ounce of difference in their ability to withstand heavy use/abuse over many years of enjoyment.
Again...I absolutely don't think you can go wrong with either one for a high quality product, but the edge goes to Seekins for me.