Pima cotton is supposed to be softer, Egyptian cotton is supposed to be smoother. Ours are pima, only 275 thread count, but they're 5-10 years old and REALLY soft. (Royal Velvet, before the company went under and sold the name.) I've been tempted to buy some higher thread count egyptian cotton sheets to try, but the sheets we have feel so good that it's hard to spend the money. We bought some Calvin Klein's recently because they were pretty. Don't recommend them--very scratchy.
For flannel, we have both 5 oz and 6 oz Lands' End. On cold nights, they're great--you get in the bed and it's not cold. (Don't know how the new "anti-wrinkle" line is tho).
One thing we have noticed--the more dye (i.e., the more color or pattern) to the sheet, the less soft it seems to be.