Posts and comments are almost identical from the perspective of the blockchain. Each is a transaction that is being submitted and that costs are almost the same. In some ways posts have more cost (they are usually  larger) and for other technical reasons, with the same content, comments can have slightly higher cost.

Trying to put timers on accounts as a mechanism to control abuse does not work. The abusers will set up multiple accounts and round-robin between then to evade the limit. At the same time, responsible non-abusing users won't do this and will be the ones whose experience is made worse by the limits (which is exactly what we saw with the previous 30 second limit on comments).