Depends. Slots work really well for games like resident evil where there's less items in general, but you want the player to still pick and choose what they take with them. This will usually make the player consider close to every item they pick up, and they'll be messing around with their inventory more often. Which is ok because there's less items in general and you can tell at a glance what fits and what doesn't.
Numerical weight works better for games with more items like RPGs (especially dungeon crawly ones) because you can just pick things up as you go and only consider what to leave behind every so often when you hit the weight limit. You'll still have to ignore some items sometimes, or risk giving up a healing item in order to earn more by cashing in more treasure, but you won't (or shouldn't) be constantly going in and out of the inventory.
Of course both of these generally work better in games that have more linear or traditional level design. So many games going open and expecting you to be out in the world for longer periods, weight systems of any kind skew more towards useless tedium rather than meaningful pacing or decision making.