One: The F1ultralight is slimmer than the F1, by a lot. On paper, the difference between 3/4″ (19mm) and 1″ (25.4mm) wide webbing might not sound that big, but it is — and the hardware is smaller too. The sum of the F1ultralight’s parts is significantly more compact, even though the length and range of adjustment are practically the same.
With very small cameras, there’s no appreciable difference in comfort (we recommend the F1ultralight for cameras up to 1.5lb [.68kg] even though it’s fully-capable of supporting much more), but you may find an F1 more comfortable on bigger cameras (wider webbing distributes weight across a larger area).
Two: The F1ultralight employs a handy “rip cord” which can be used to form a lanyard/long wrist strap (or fold the strap neatly for storage) — whereas on the F1, the webbing is looped through the D-rings to form a wrist strap.