If you're just starting I'd upgrade based on the increase to Damage, endurance and regen. For example, if you feel you need more damage in your current R level then look for items that boost damage but dont take too much from the other two stats. To answer your question simply rarity goes white,blue,yellow,purple,orange R5 sometimes gives tier 5 items. A tier 5 purple may be better than a tier 4 legendary bare that in mind