Here's my take, assuming same number of miles. First of all, a higher Wh/mi rating means the battery pack has had more charging cycles, but the better question is what is considered a normal range. Second of all, it's impossible to tell by that figure how the car was driven; could have been hard, fast, short trips with HVAC on, when cold, or a combination of any or all of those. And finally, even if the Wh/mi rating is low, that owner might have 'abused' the battery by letting it fully discharge, by charging to 100% often, etc.
Bottom line is if the car doesn't have a life time meter of average Wh/mi (owner reset all trip meters), a lifetime charging graph with minimum and maximum %, and other such meters, it'd be impossible to guess which is the better car to buy. So if I was buying used (I don't buy used), I'd rely on the same system I'd use with ICE cars: buy from a mature original owner, and ask all kinds of questions, and you should be able to tell how the car was used. I have to say the great thing about EVs is you basically don't have to worry about the motors on the mechanical side of things; just the battery pack, and how it was treated.
