While it's true that more than likely it will only be 12g wiring behind the wall, I still like 10g for high continuous draw uses like our Leaf. Well actually I'd probably be OK with 12g using the stock 12/13a Leaf EVSE but my portable EVSE is capable of 16 or even up to 19a in which case I'd really want the heavier 10g.
Thinking it through, if your only going to use the stock EVSE(12a) a decent 12g extension cord up to 50' should be fine. If going to 100' maybe 10g might be warranted but 10g is much harder to find and costs significantly more, oh and is heavier/bulkier too. AFA where to purchase one, Costco or if you have a Menards near you would be a good place to get a decent 12g 50' cord, Menards and possibly Home Depot should sell one 10g 50' model but expect to pay 50% more than a 12g model. Costco won't sell 10g.
Not sure where you live but in my area one of the most important thing about an extension cord is how it handles cold weather, the cheap orange ones(generally only 16g or maybe 14g) get so stiff in below freezing temps they are almost impossible to coil back up. The yellow or blue are better suited for below freezing and for sure below zero temp use.
Oh one nice feature is if the female end of the extension cord lights up when power is present, makes it quick and easy to check if the outlet has power.
Edit, I see your in NY, I guess it can get cold in NY so I'd really avoid the orange non-cold weather rated cords, they are crap in cold weather!
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a