My own experience would say start with the MKR kit.
The projects were made to be compatible as they are mostly simple projects designed to get you up to speed in all the basics by following the book.
Once you exhaust the book then you would no doubt be full of ideas on further things to try.
The MKR is also breadboard friendly unlike the UNO starter kit.
The price points are very comparable (minus the wifi shield)
but you would be left with a faster board capable of so much more than just a UNO.