Avman, do you have to laboriously type in full instructions, or is it automated to a degree (eg click on flight number, then 'descend flight level' icon, then type in '110 for a descent to flight level 110)? If the latter then I would imagine that would with practice prove quicker than speaking, as you could get the instruction out virtually as quickly as saying it, and wouldn't have to stop whilst the pilot repeated what you have said. Typing it in full on the other hand (especially if you use as few fingers as I do) would slow things considerably I would think.
The real fun would happen in the circumstances that witchdoctor describes - the same discussion as that of hand flying vs automatics that pilots have would be valid.