I haven't used SF, but we do use GWG and it's the first grammar program we've actually completed, let alone ordered for the next year. Looking at the 2nd grade SF samples online, I'd say they are fairly similar in style and, in general what they cover.
SF covers more style related writing topics, particularly writing for tests, whereas GWG covers homonyms, synonyms, antonyms, dictionary use and writing a friendly letter.
GWG has two pages per lesson, three lessons per topic. In the 2nd grade there are 95 lessons, plus three review lessons at the end of each topic (i.e. verbs). There is no test prep, presumably because the Tamela Davis, the author, was writing for a homeschool audience.
I really have no idea who would expect a second grader to write the pieces SF has, or be cognizant of active/passive voice, or using a scoring rubric to determine whether an author's writing is good. Frankly, those exercises (minus the scoring rubric) were the focus of English 101 in college, and can certainly be addressed much further down the educational career than early elementary school, with kids at an age where holding the pencil and putting something legible and fairly accurately spelled is a challenge. (we will ignore for the moment the rest of the rant about how the grammar stage is for soaking up the rules, not honing self-expression :D)
I would say that GWG is the gentler approach, and I think it covers more.
Aside from my jumping up and down, did that help at all?
Peace,
Angela