I prefer carbide to HSS for a few reasons. They stay sharper, longer. They stay sharper, longer, and they stay sharper longer. I have not experienced smoother cuts with HSS because I no longer own any. One thing I will tell you though about carbide spiral bits. They are generally solid carbide, not just the cutting surface, bit the shank and everything. Carbide is a very hard, very brittle substance. If you suffer from fumble fingers, be careful. If you drop them, they quite often break, or even worse, crack. The cracks are worse because they tend to go un noticed, and then break when you start to work, which damages your workpiece.
Down cut bits generally work well then you are doing through routing, (They throw the waste down, and away from the router, which keeps your view of the cutting path cleaner. Upcut bits are my favorites for sutting mortises. They produce a nice, smooth cut, and don't tend to clog up in tight spaces because the chips tend to lift up and out of the mortise as you cut.
If you are doing pattern work with a guide bushing, or bearing, remember that an upcut bit will leave a little bit of a fuzzy edge on the very top corner of the material, and the opposite for a down cut bit. This fuzz is easy to remove, but you may want to keep this in mind when you are choosing which side of the wood will be the finish side of the stock.
You won't be disappointed if you spend the money for these bits.