Aluminum Tube
There are two possibilities for the fuel pickup tubes, the sawcut ones as shown in the plans and the optional ones in the accessories catalog online which have a finger screen safety wired and prosealed to the tube. Consider the latter for your tanks. I think the type of sawcut/screen is more important than the type of tubing.
In answer to your question, yes, stainless steel is much harder to flare and bend. If you want a better material than the soft aluminum tube in the kit consider 5052-0 in the ACS catalog, it is a much stronger alloy. In any case, whether 3003-0, 5052-0 or stainless, the material should be annealed or
"-0" temper for easier fabrication.