I prefer old-time, because to me bluegrass seems more structured. I hear many variations in playing oldtime music in jams, but it is subtle. Also, oldtime players tend to play more in unison, and not take "breaks" as clearcut as bluegrass players do. Oldtime is more informal to my ear.
Here in Kentucky, there is a very real differance between oldtime and bluegrass. Of course, there's room for both!