Off hand attacks do not rely on main hand attacks to 'proc' it's completely separate timers.
So with that said the main consideration is purely ratio, except for a few minor things:
1. Lower speed weapons will give you less ripostes and damage shield damage.
2. Faster speed will give you more ripostes but also give you a higher chance to interrupt.
3. Stats and or procs. But these are rarely worth giving up ratio for.
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