To understand what is gameplay itself, you do not have to buy. Demo / web version is enough. All versions are pretty similar. So if you do not want to donate/support development, just do not buy.
Web version has the same number of the levels. The difference only in background soundtrack looping implementation(vm implementation has a bug in sound/music looping) and the absence of the requirements in story mode sequence of the levels(you can choose last implemented level independent on progress, instead of win step by step)