I use a Havel's that is a scalpel-style with a curved blade.
I also use snips - those little purple-handled snips from Connecting Threads. The points are VERY pointy so I can get under the thread and just snip every 5th stitch or so.
I also like the very ordinary little blue seam rippers with the clear caps, but they seem to go dull VERY fast.
I use them interchangeably - whichever is handy and whichever seems to do the job best. Sometimes I start with one and then switch to another - not sure if it's my mood or different fabrics or what but sometimes one seems to do the job better than another.