Setting up a landing page and doing a few basic things to draw attention to it should take you a day or less. You can either spend that time right away and have 500 people that care when you launch, or you can spend that time when you finish your MVP and launch and have no one that cares.

It's time you have to spend anyway, do it early, let it sit there and attract interest.

One thing you can't do when you finish your MVP is go back 3 months and have a landing page that attracted interest.