I have just completed geometry, and I am taking Algebra II next year (though, I taught a lot of the topics to myself on the side during geometry this year). Algebra II does not really involve a lot of geometry. Sometimes, depending on your school’s curriculum, you may learn vectors and conic sections in geometry and then continue on with it in Algebra II (though, not many schools seem to introduce those topics in geometry, generally Algebra II). Some schools have students take Algebra II concurrently with Geometry if need be, so I don’t think taking geometry over the summer would really predict a student’s success in Algebra II.
Algebra II builds more upon Algebra I. For example, in the class, you will work more with parabolas, factoring polynomials, and exponents (logarithms will be introduced. You will learn cube roots and complex numbers. These ideas build upon what you have learned in Algebra I (NOT GEOMETRY), so again, success in geometry is not really determine whether a student will be successful in Algebra II because they are so different.
My biggest concern with you taking geometry over the summer is with the SAT. There are a lot of geometry-based problems on there, and taking a condensed version of the class over the summer may put you at a disadvantage. Assuming you took the SAT later in the school year, you would have a pretty solid understanding of the concepts learned in geometry, which may help you on the test. Does your school allow you to take geometry concurrently with Algebra II? If so, that’s what I probably recommend doing (assuming you would be comfortable with 2 math classes at once).
If you do decide to take geometry over the summer, that’s fine, but please just make sure you really understand the concepts well enough. I would advise taking a summer class and also purchasing a geometry book that could be supplemental to the course. Since geometry is a condensed version of a class, you may not get to all of the topics over the summer. Below, I have the topics that are generally omitted from the summer version of geometry, and if not covered, I recommend learning these yourself for the SAT:
*Similar Right Triangles
*Simplifying Radicals
*Cross-Sections of 3-Dimensional Figures
*How to Find the Area of Any Regular Polygon (A = 1/2aP)
Unless you intend becoming a math major, I think it may be too difficult (and maybe stressful) to try to catch-up in math. Do whatever you feel comfortable with, though, and I wish you the best of luck with whatever you choose,