This project is coming along greatly. About the gameplay/gfx debate:

Most probably you wont have AAA style advertising - knowledge of this will get out by mouth, which means you must have good gameplay so that what people will say about it is good.

It's an indie game - not many people will upgrade to play this. Also, there will always be better AAA games in terms of GFX... an indie game in my opinion must get an audience through solid gameplay.

However, most people care a lot about GFX, and its the first thing they see - First impressions are important.

It's free (I think). The ratio of people who know about it and have played it will be closer to 1:1 than an AAA game. More people who know about it will know its gameplay. Get solid gameplay.

The choice is yours, ofc both is excellent, but take these things into account (unless I'm wrong).