The 2017 will qualify for the rebate and the 2015 will not so make sure you are factoring that in your cost equation. Next, do both cars have all of the features you want?
Next is the cost to operate. The gen 1 will go about 40km winter and 60km summer and then start using premium gas at about 6l/100km. The gen 2 will go about 30% further on electricity and then start using regular gas at around or slightly better fuel economy then the gen 1.
If your commute is greater than 40km round trip, the gen 2 starts to make more sense. If you want a heated steering wheel, rear heated seats, have more than 2 kids, gen 2 makes more sense.
If none of that matters and the gen 1 is at a super deal, then it would be worth considering
In Canada, used Volt prices seem to be high.
What access do you have for charging? The gen 2 will take longer to charge so 240v becomes more obvious. If charging on 120v, the gen 2 can be set to automatically use 12amp level charging while the gen 1 will require you to manually set to 12amp each time.
The 2015 will be considered a 2 year old car already as it is so close to year end. If you are leaning that way, make sure the price you pay factors that in.
There is the comfort factor as well. The two different generations are slightly different in comfort.