Threadless or threaded headset

You don't normaly get to pick. The frameset will dictate the headset you can use.

usually true, but he wrote:

planning on buying an actual "track" frame (I know it's not a genuine track geometry, but still...) - either the IRO Angus (http://www.irocycle.com/angusframeset.aspx) or the Mark V Pro (http://www.irocycle.com/markvproframeset.aspx)

and if you take a look at the specs youll see that really the only difference seems to be threaded or threadless