I think that the issue of spirit vs spirit here is more than just one dimensional
obviously we have shanks who floors people with his presence
but then again, luffy never showed fear of someone before. he might have been concerned certain techniques were gonna hurt him, but he was never intimidated by someone's presence
also, there is a question of the context of their meeting and whether they would be harboring presence towards each other that would be intimidating. shanks and luffy are close friends, especially since shanks is something of a mentor/father figure to luffy
that being said, if we were to ignore all of those dimensions and just look at spirit vs spirit, luffy has shown some crazy spirit when he finds the right reasons in a similar way to shanks. it is really measured by their conviction in what they are seeking whether its to kick crocodile's ass or to stop ace from getting himself killed. as far as that goes, there is no objective way to measure it.
but be assured that as far as fighting goes, luffy in 2nd & 3rd gear combined might STILL not match up to the experience and ambition of shanks