Many thanks for you comments on the SP4, very interesting
The only thing I can see you leaving out in your thought process of options is potential and I presume you already own a smart phone (IPhone ?) which is already covering most functionality of an Ipad and giving you a partial Apple fix
Once your in the <2lb category I think there little argument on portability difference other than the iPad Air 2 LTE will be much easier as a couch reading device than the rMB, IPP,SP4 etc
As already noted if you have an IPhone an Ipad is duplicating a lot of functionality you already have and the LTE iPad Air 2 carried other costs etc for the data plan and just adds to the headache on sims if you travel internationally.
The full OS rMB or SP4 give you the greatest flexibility when away from home be it plugging in to your hotel TV or friends with files/movies on a thumb drive, synching with cameras etc etc, let alone the bigger SDD drive as your UL/DL speeds wont be the same as when at home
Your SP4 mirrors very easy to your TV and it is also possible on the rMB with Airplay and setting up the IP addresses so you still have the functionality of the Mac Mini but with more options including iTunes/IMovies etc on both OS's
I think the flexibility, connectivity, superior software and a multitude of options open to you by having your smart phone and a rMB or a SP4 is a negligible compromise compared to a Mac Mini + iPad Air 2 LTE Combo + smart phone and especially as it seems your not a collector or major fanatic of 100's of apps