Well it is a very interesting discussion as to what is best indeed. In my case I already have a Prius plug in. My driving in it is 80% EV even though you only have a EV range of about 14 miles (12 to 15 miles actually). I bought the i3 Rex even though most of my driving: as I am retired, is going to be within the range of a BEV except living in the mid west the charging infrastructure is really spotty. My i3 will be used mostly on trips in the area well within 40 to 50 miles each way, so I got the rex because many times I would have to charge up just to get home, yet the rex will usually get me home needing to only contribute 5, 10 or sometimes 20 miles past the range of the BEV.
I also plan on keeping the Prius, as we like to go on vacations of 600 or 700 miles each way and with the current infrastructure for charging I would never want to drive the i3 Rex that far the way you would have to recharge the Rex/gas every 60 or 65 minutes, it just isn't piratical or even safe in my book, because of the way the Rex is set up in the USA. If we had the European Rex operating configuration it would be no problem in my opinion to go much further on the REx, but in the US as currently designed it is not safe.
Now having said that, I bought the DC fast charging as I think infrastructure in my area will probably start be built much faster now. Additionally I am hoping BMW US will eventually fix the Rex functionality and then I woill be willing to take longer trips in it. In the mean time long trips will be reserved for the Prius.
So I agree with ultraturtle, at least for me the Rex will get me more EV miles overall in the i3 even though I will still have a ICE/plugin hybrid that I can take on long trips.