if you are planning to remove the wire (I recommend) then it does not matter much what you use.  My experience is that aluminum wire tends to break easier than copper, that said, you really should not have enough strain on the wire to be an issue, if it is not fitting right, ususally an indication that something needs to be shaved/shaped.  If you are building a WD, the bow & stern have a lot of pressure, I added a couple extra wires, it was covered by my Carbon bottom so did not have to worry about extra "ugly" holes.

I think that the copper wire is very forgiving. Remember that you may be re-adjstung the tighness on some of the wires to fair the hull before epoxy tabbing or filleting.  I am not sure that the aluminum wire will take the tensions or re-adjustments that the copper wires will take. Once you have over 100 stitches in the boat and find out if the aluminum works or not you may or may not regret the choice of saving a few dollars.  Good Luck.

i am currently using .032 stainless steel safety wire [because i had some and the copper is very expensive].  it is stronger than copper and i'm liking it better.  you have to learn however to reach carefully, because the s/s wire ends will poke a hole in your finger very easily being stiffer than the copper.

Used the aluminum wire to stitch boat panels. I think it worked pretty well. You have to get used to it though. tensile strength is fine it is the sheer strength that is a little weak. You will snap a few wires when twisting them if you go too tight. You do not want to overtighten them anyway. The wire holds fine on the ends of a sasafrass 16 and there is quite a bit of pressure on those ends. I would definitely use the aluminum wire again. 18 gauge aluminum wire comes in 50 foot rolls. I found it at a few different home improvent stores for around $3.00 a roll. Used four rolls for a sassafras 16. Little savings here and there can add up to a hundred bucks or so by the end of the project. Don't skimp on materials, but save where you can.