The X is the newer model so it has USBC charging (in and out) and apparently better lithium cells.
The older model has slower USB charging but also 12v in and out.
Both a GB70 and GBX75 will do the starting it depends what else you find useful as a power bank. If that's not interesting maybe get the cheapest (likely GB70), if you want to power 12v devices go GB70 if you want to power USBC devices go GBX75
@Dellmassive is the NoCo guru so may have an opinion either way.
If you have Amazon Prime might be worth waiting until next week as 16/17 is Prime Day and there are often offers on NoCo kit - but make a note of the prices today to be sure you're getting an actual bargain.