If you're still on the fence about whether you should take the SAT or the ACT, or if you've been prepping for one test all along and are thinking of taking the other one, here's a very general overview of what you should know about the grammar for each exam:
The SAT places a very heavy emphasis on the following:
-Verb tense (particularly various forms of the past tense)
-Modification (dangling and misplaced)
And tests the following frequently as well:
-Idiomatic phrasing (prepositions and gerunds vs. infinitives)
-Adjective and adverb usage
-Logical relationships between clauses (conjunctions)
-Relative pronouns (who, which, in which, etc.)
Diction and Redundancy problems are tested rarely.
Punctuation is covered only minimally: the only two punctuation marks tested are the comma and the semicolon. The former is only tested in regard to using coordinating (FANBOYS) conjunctions to join independent clauses, and to creating non-essential clauses within a sentence. The latter is tested only in regard to joining independent clauses and before certain conjunctive adverbs (however, therefore, moreover, consequently), never to break up excessively length sentences.
Dashes, apostrophes, and colons are not explicitly covered.
For the most part, the SAT tests various grammar rules in isolated sentences. Only in the Fixing Paragraphs section (6 questions) does context come into play. While this does make SAT Writing simpler in some regards, the sentences are also more complex and contain more constructions that high school students are likely to be unfamiliar with (and thus likely to erroneously believe are incorrect). The "No error" option, particularly in the Error-Identification section, means that many test-takers find this section of the SAT to be very tricky.
The biggest difference between the SAT and and the ACT is the format. All ACT questions are presented in the context of a passage, forcing students to pay close attention to context and to employ both reading and writing skills simultaneously. An answer that is grammatically correct may thus not be the right answer if it is inconsistent with the style or or tense of the surrounding sentences, or does not logically fit in with the information presented in the paragraph or passage in which it appears. In comparison to SAT sentence, which often deal with literary or historical themes, ACT passages tend to focus on more quotidian matters.
Strictly in terms of grammar, however, the ACT is a more punctuation-oriented test than is the SAT. Commas are not only tested extensively in relation to independent and non-essential clauses but also in relation to dependent clauses and adjectives.
Test-takers are also required to identify the incorrect use of commas in many more ways than on the SAT: between compound subjects and objects, between multiple adjectives, before prepositional phrases, between subjects and verbs, and before relative clauses with and without "that" (usually the most difficult for test-takers to identify).
Other punctuation marks tested include the following:
-Semicolons (to separate independent clauses and before the conjunctive adverb "however")
-Colons (before lists and explanations)
-Dashes (to set off non-essential information and explanations)
-Apostrophes (its vs. it's, etc.)
Common non-punctuation concepts tested include:
-Subject-verb agreement (only a couple of questions per test at the most, usually in the form of subject-prepositional phrase-verb)
-Verb tense and form (often context-dependent)
-Adjective vs. adverb
-Dangling and misplaced modifiers (only a couple of questions per test)
-Clarity and conciseness
-Diction and register (too casual vs. appropriate for a formal piece of writing)
In short, there is actually a substantial area of overlap between the two tests. Each has its own quirks (some people who are used to taking the SAT find the constant back-and-forth of the ACT disconcerting at first), but the majority of the material covered is similar. Deciding which test to focus on is ultimately a matter of figuring out which one feels more comfortable.