classes.
items have the same inventory structure as classes but since you pick each one manually most players will lose out on good item synergy because they don't fully understand the game. the ones that do understand will obviously pick the best item combinations every time because they've minmaxed all the advantages and disadvantages of the weapons combos.
in a class structure you as the designer know the synergies of the game (since you designed it in some way) and therefore you can save everyone the trouble of having to discover the best item combos by giving it to them pre-made. it saves time for everyone except you as the designer who has to find and implement the synergies. it also allows you as the designer to visualize the synergies in an abstracted form and it will make it easier for you to tweak the classes to nerf/buff or emphasize the advantages of the items
in the end spiritual 'classes' will exist on the server, since all players will eventually know which items work with each other and always select the best combo. either you design the classes yourself or the playerbase will do it for you. the items that don't synergize will eventually just be underused
all of the above is my experience hosting class trench wars and tournaments. as far as gameplay goes, class trench wars was definitely more consistent and fun to play and design, while tournaments was a pain in the ass for everyone because it required so many isolated tweaks to the items