If your CTO is about to leave... somebody will have to materialize somehow what the CEO sells.
I think CTO job (technology) can be replicated with sufficient resources ( time, money, and effort) but I am not sure it your company has enough {time,money} to make it happen.
The same could be said for a CEO; but my intuition (which could be wrong) is that for a startup a CEO is easier to replace than a CTO.
