A thing that helps me distinguish them is that Helvetica is a bit more decorative with some of the capitals, like G and R. I think the Arial designers were probably trying to make it "even more Helvetica than Helvetica", by simplifying the flourishey/decorative bits, like the bottom thing on a G and the curvy bit at the top of the slanty line of the R. (These are the proper typographical terms, btw.)