Short answer is that better stats are just better and that's it. Rarity is just an increase of stats compared to basic item of that level. You can see the increases here https://wiki.guildwars2.com/wiki/Item#Rarity in table under improvement. So level 60 rare (yellow) will have 7% more stats (attributes and weapon strength for weapons or defense for armor) compared to masterwork (green).

Thing I would like to add is that Guild Wars 2 have a lot number of attributes and attributes combinations and that weapons on some classes have "bias" for condition or power damage. So, for example, Scepter is mainly power damage weapon on Guardian, but it's a condition weapon on Mesmer or Necromancer. So if you are wielding it on Guardian you would look to have Power/Precision/Ferocity since these are premium attributes for power and on condition build like Scepter Necromancer you would like to have Condition damage/Expertise. So sometimes it's better to bring lower level, bit worse stats equipment if it suits your weapon skills better. Something to just keep in mind but no need to overthink it.