Just because you opt for the SP3R rail does NOT mean you have to get the iRMT-3 upper with the integrally machined mounting block for the SP3R. The rails come complete with the adapter block that will allow you to run the SP3R with almost ANY standard AR-15 upper (billet or otherwise, including Seekins' own SP223 billet upper). That will give you some modularity down the road if you decide you ultimately don't care for the SP3R or you need to take your rifle in a new direction that does not include the SP3R.
As for which one...it really all comes down to an matter of personal preference as to which rail best suits your particular needs/wants in terms of features, etc. I have used both and find them to be quite different. The SP3R wins hands down for me based on the shape/size of the rail which really lends itself to improvised shooting positions given the flats along the sides and bottom of the rail which you won't find on the Geissele. The newer SP3R is setup for all manner of keymod accessories/rails/etc., which the Geissele is not (yet), which leaves you with some limitations (not a HUGE deal, but a big difference nonetheless). Both are very easy to install, but again, the edge goes to the SP3R (especially if you go with the iRMT-3 upper) as all you need is a 1-1/4" crowfoot wrench with whatever size drive matches your torque wrench to install the barrel, then chuck the rail on and a couple of torx screws later...you are DONE. Similarly though, once you get the Geissele proprietary nut on, it is as simple to remove/reinstall with only two screws and the indexing screw.
