It has been awhile for me too
As vash demonstrated, as long as the steel specification is the same, I think the 7x7x.375 is your stronger section...
However, you might want to look at how you are attaching your section to the pivot...If your weak point is where the connection (i.e. bolts or weld) are, then you might be better off with the 8x8 due to bolt spacing, distance of bolt holes from end of section, or length of weld.