The slotted is for use with a float system for really big fish. Mostly salt-water guys stick an arrow in a ray or shark and let the fish take the arrow, some line and an attached float. That is what the slot is for...so the line can slide out. Here in Ohio, I've never had the need for floats. In fact, I have seen the slotted reel be a big pain in the butt while shooting because the line tends to get caught in the slot and makes it a pain to reel...even if the slot is taped over.
Bottom line: unless you plan to go after really big, tough fish the 310R is all you need. I love my lefty version. I wouldn't shoot fish with anything else.