Our Forms series and Henle I both teach the Latin grammar. The difference in the Forms is that the students are given more practice with the workbook that contains 5-6 pages of exercises per lesson. We are moving at a slower pace than Henle I because we are focusing on mastery and lots of review and reinforcement, regular oral recitations, etc. So, when you finish the Forms, you are really ready to move into Henle II or another translation course of your choice. The Forms especially work better with younger children because of this slower-paced concentration on mastery. At our school, we begin Latin in 2nd grade, so our students would be ready for Henle in 4th grade, and that was just too young for Henle. For a high schooler or college student, either program will accomplish the same goal - a grounding in the Latin grammar. Many homeschoolers choose the Forms because we give the teachers help with our scripted lesson plans, dvds teaching the lessons, online classes, etc. It is much more user-friendly than Henle.
I hope this helps.