Is it the buyer or sellers resposibility to register the land contract?

Depends on local law. In many areas a land sales contract doesn't need to be registered. If this is a normal purchase, the contract for sale doesn't need to be registered.

However, since the advantage of registration of a land sales contract is weighted heavily on the buyer, the buyer should make sure its registered.

I will agree, where is this located at. In most areas they do not have to be, and I have bought and sold many properties on contract and never registered any of them.

The buyer is the only one protected really by registering it, so the buyer should do so, if he desires it to be done.
Unless local law requires seller to do it