Well for one, you can run heavier line on a light weight baitcast reel. Versus a big akward 4000 Shimano spinning reel that gets heavy if you fish for more than 5 hours with non-stop casting, or if you're a tournament fisherman. For example, the Zara Spook and other 3/8-1/2oz topwaters require 12-14lb line to get the lure to work right on the surface. Also for spinnerbaits heavier than 1/4oz, you're gonna want that heavy 10-12lb line, spinning reels don't handle heavier lines as well. You wouldn't want 12lb test on a 2000 size shimano spin reel. Spinning reels cast better with limp/thin line. There's your flick of the spin cast, then you have your hard chuck you get from the baitcaster. Sometimes you need to chuck that __________ out there into the wind with a big lure, for that, you're gonna want baitcast. For finese fishing clear water, or small size ponds, you'll appreciate the *blip* landing the spinning gear gives vs. the *SPLASH* of the baitcaster.
Good rule of thumb: If you mostly use 1/4oz and lighter lures, and you don't fish heavy cover, stick with spinning gear. It's when you start using the big topwater lures when you need the baitcast reel. Or when you start getting into casting the deep diving crankbaits into tough underwater structures, rocks, weeds, etc. Are there a lot of trees along the banks from where you'll be casting?...stick with spinning reels, you can flick your lures underhanded greater distances with ease. On a light or med. light spin rod you can build up a spring bounce for the cast, to launch it out there, that's the main difference. Baitcasters need room to swing the rod in more than a 90 degree angle. You don't need to swing the rod that much with spinning gear = better for casting from confined spaces.