Is the Plural of Cactus "Cacti" or "Cactuses"?

Both are just fine, as you can verify in the dictionary. ("Cactus" is apparently also standard.) Cacti appears more common, judging both by a Google search and a Westlaw ALLNEWSPLUS search; but cactuses is clearly fully standard.

I generally prefer to use English-formed plurals like "cactuses," rather than original-language plurals. On the other hand, others might prefer to use the more common term, partly because they worry that some readers might wrongly assume that "cactuses" is wrong. And if you're writing for a specialized field (for instance, botanists), it's generally safer to use the term that's most common in that field (I don't know which that is, but I take it that botanists do).

But in any case, there's no basis for treating either term as an error. As with many either-or questions related to usage or pronunciation, the answer here is "both."