If you are okay with the limitations imposed by their design philosophy of purposefully not giving users flexibility to choose their own fonts or colors, and you don’t need the more hardcore writing features of Ulysses then go for it. But in my opinion the apps are so different and are really different types of writing apps. If you consider iA Writer similar enough for your limited needs you should really be comparing iA Writer to different Markdown editors, like MWeb or Typora (free for years now in endless beta, to be a commercial app eventually), or Byword, or Quiver, or even Bear.
Ulysses has far more features and flexibility and customizability. If it seems equivalent for you that’s great - I own it for Mac and iOS and use it occasionally (more on iOS these days), but I could not imagine living with one of only two imposed screen modes and one of 3 imposed open-source fonts, and no customizable colors for text or background. iA Writer looks good, but it gets boring for me and I need more customizability
iA Writer (light and dark modes):
Ulysses (customized light and dark modes and font):
BBEdit (customized colors and font):
I spent many years inside the BBEdit text editor and I have decided preferences for font choice and color background. The devs at iA are ‘opinionated’ but too much so in my opinion.