By the power of truth, I, while living, have conquered the universe.
Both have the same aesthetic, but Pilot is much thinner where as SPI is actually quite bulbous. I hate that thing on the mouth-piece of EVA in Reach, what was the point in it? Looks like a buck-tooth.
I still go for Pilot, it was my every intention to wear some kind of SPI-esque armour. Shame Bungie didn't include it, Headhunters reveals that there is a MJOLNIR version of SPI and people have obviously wanted it since early screens of EVA in Halo 3 came out...