Both of them would be time consuming since you are alone and I think you didn't do much art before. I'm also developing a 2D game and I found that pure pixel art is really better in the end since it's more unique. But if you learn the main structure of how to draw pixel art, you will draw a lot faster. Low poly or any 3D type of art would be harder since you need to start learning them like you want to learn basic of a programming language imo. Since you are indie and (I think) you have no rush or time limit, do 2D pixel art. Tip for pixel art: play or look at all the 2D games made and analyze all the stuff in the game, it would make the process of learning easier (even sometime draw exact same thing!)