HI...the LARGEST difference is the battery (which is the ultimate purpose of this car) ....where the ELR 2014 model may get 38 miles on a charge...however the 2017 Volt with the larger battery is stated at 53 miles and some users are reporting up to 60 miles on a charge? There are many other user friendly differences in tech as well as body style and utility.
For me its all about the EV miles but that is subjective to what you want? Like? and need??
good luck in your choice