The free version of Isotoop works already perfect. With Pro you have more options.
1 More colors options
Pro has more colors for blocks, modules and typography.
Free has 2 colors in 13 tints: red and grey.
Pro has all 19 colors in 13 tints.
2 More typography options
Pro has more styles and font sizes.
Free has 2 font sizes: t-n (normal) and t-l (large).
Pro has all 6 font sizes: t-xs, t-s, t-n (normal), t-l, t-xl and t-xxl.
3 More structure options
Pro has more options for spacing and structure.
Free has 2 spacing values: Normal and Large
p-n (padding normal), p-l (padding large)
m-l (margin normal), m-l (margin large)
Pro has all 7 spacing values: p-0, p-xs, p-s, p-n, p-l, p-xl and p-xxl.