6ga costs more of course but if you might ever use a second length added on then opting for 6 is a very good idea.
Now as regards the hocus pocus about wire changing resistance because it is in series with a different ga wire. One word, BALDERDASH!!!
Resistances in series add arithmetically and do not change because they somehow know what they are in series with. I have two extension cords for my Lincoln AC-DC Tombstone. One is about 66 ft long and the other is about 33 and were made by cutting a used one I bought into two pieces. If I am close I can use the 33 if farther, the 66 and farther still, both. I can run the welder at full power all day with the cord in the sun and not have a problem (except my personal comfort/dehydration.)
The used cord was some sort of multi-phase cord, 3 phase I think. Multiple conductors, not all the same gauge. I left some conductors to stand alone and paralleled others and ended up with a 240 VAC cord with both legs plus neutral. The female ends are metal J-boxes with a 250 volt 50 amp (old welder/dryer style to fit the Lincoln) female on one side and a pair of duplex 120VAC outlets on the other side to run tools, lights, etc.
I got some funny comments about not having perfectly matched conductors for both legs of the 240. Again, balderdash! In a 240VAC circuit such as my Lincoln tombstone the extension cord wires of both legs are in series with first one wire supplying electrons and the second taking them away and then reversing those roles 120 times a second due to 60Hz AC supply.
There are problems related to using multiple extension cords. All of the cords must meet the ampacity requirement irrespective of their length. If you have a cord rated at 65 amps and another (irrespective of its length, even really short) rated at 40 amps and you use them in series you will over heat the lower rated cord if you exceed 40 amps. Using cords in series you must have both cords as heavy gauge as if it were a single long cord.
OK, I'll drop the other shoe... another problem with multiple extension cords is another opportunity for a less than perfect connection. Be sure to get decent quality connectors and make sure you get a good tight connection when joining cords.
The other drawback of conservatively sized extension cords (heavy ga) is they are heavy and tiring to carry very far but that is one of the costs of doing business conservatively, safely, and efficiently. Undersized extension cords rob you of arc power, make welding harder, and waste the electricity you are paying for in heating the extension rather than the arc.
None of my BALDERDASH comments are intended to hurt anyone's feelings. If you are not conversant with P=IE and I=E/R and all the algebraic manipulations of same then debating electricity with you would be a battle of wits with an unarmed man.
Pat