In this case, they are the same # of stats though, but equally good. If anything, the haste (imo) would be better than the spirit, especially seeing as it has another upgrade. It's like the glove enchants too:
+50 haste or mastery is a personal choice as a holy priest. Stacking either is good. Is it just based on there being another upgrade available? Because that doesn't always reflect what's best/equally good for the time being then. I guess it's weird because I could go and boost my personal score by getting enchants that aren't necessarily good for me when I prefer mastery over haste and haste over spirit. Seems the +50's should all be worth 18 in these cases.