Generally speaking, quartz countertops tend to have a better seam and go together more closely and easily. This is partially because they're easy to cut and the patterns are manmade, so they can be carefully planned out so that they match.
Nonetheless, opinion is divided on this. Lots of kitchen installers say that quartz seams look more obvious because the uniformity of the quartz makes the seam stand out a lot more.
On the flip side, granite countertops have naturally random veins and patterns that draw your eye away from the seams, but it's basically impossible to “match” one slab of granite with another one.
So there's no easy answer to this question. Technically quartz will fit together more snugly and create a tighter seam, but it may look out of place. Granite will fit together awkwardly but the natural randomness of the stone distracts from the seam lines in the first place.
It all depends on the look you're going for!
See More Countertop Comparisons Below
Quartz ComparisonsQuartz vs MarbleQuartz vs LaminateQuartz vs Solid Surface
Granite ComparisonsGranite vs MarbleGranite vs Laminate Granite vs Solid Surface