Google Books searches reveal various competing expressions that begin with "content is better than"—including ones that refer to each of the two main senses of content: "contentment" and "substance."
'Content' as contentment
By far the most common of these aphorisms is the one that Julie Carter identifies in her answer:
Content is better than riches [or wealth].
According to W. Gurney Benham, A Book of Quotations, Proverbs and Household Words (1907), this sentiment is sometimes expressed more grandly as
The greatest wealth is contentment with a little.
James Moffat, A New Translation of the Bible, Containing the Old and New Testaments (1954) contrasts content not with riches but with toil:
Still, one handful of content is better than two hands full of toil and futile effort.
And Thomas Holcroft, The Noble Peasant: A Comic Opera, in Three Acts (1784) has this, spoken by a character from whom homilies spout like water from a bubbling fountain:
Content is better than a down-bed, and the stars will be obeyed.
'Content' as substance
We also have this completely different expression (though its status as a proverb is iffy):
Content is better than form.
The earliest Google Books match for this last expression appears in Report of the Superintendent of Public Instruction of the Common Schools of Pennsylvania (1900). Google Books finds multiple instances of it over the next decade, but all of them are quoting the same commentary on the proper way to teach literature. Nevertheless it seems more relevant than the others to the OP's request for a saying that distinguishes between substance (content) and superficial appearance (here, form).