6 wing spar is basically build up with a stack of 1/4 x 1 1/4" aluminum bars to make the top & bottom beams.
7 wing spar has top & bottom beams milled from solid aluminum bars roughly to the same dimensions as the 6 stacked beams.
Both are rated for 6G for their appropriate loading. Never heard of a 6 or 7 spar breaking under designed loads.
built a few RVs, rebuilt a few more, hot rodded more, & maintained/updated a big bunch more