Should I take Physics 15, 16, or 19?

Brief course descriptions:

• Phys 15a covers Newtonian mechanics and special relativity--roughly the same topics as an advanced high school course, but with a more in-depth treatment and harder problems than a typical high school course.
• Phys 16 covers more advanced topics in Newtonian mechanics and special relativity.
• Phys 19 assumes less prior exposure to physics and mathematics than Phys 16, and covers a wider set of the foundational topics in theoretical physics, as well as some classical field theory, thermodynamics, and quantum mechanics, but with less extensive coverage of Newtonian mechanics than in 15a or 16 and with problem sets that tend to emphasize conceptual thinking. 

Making the decision:

• You should skim through the first problem sets and any lecture notes that are available on the web, and then see which class is most suitable for you. There is no clear-cut criterion for deciding, but a major consideration is that of time. Phys 15a and 19 will take a substantial amount of time, but Phys 16 will take more.
• If you think there is a reasonable chance (say, greater than 15%) that you will take Phys 16, start there. If you decide to switch to Phys 15a, the transition is hassle-free (completed homework credit will be transferred). Furthermore, if you are someone who is thinking of taking Phys 16, then the first few Phys 15a lectures will probably be mainly review, so you have little to lose by trying out Phys 16.
• As far as Phys 19 goes, a slight complication is that the topics in Phys 19 don’t line up chronologically with Phys 15a and 16, so it’s a little more difficult to switch between Phys 19 and Physics 15a/16, but still very doable. Note that Phys 19 meets on a different schedule from 15a and 16, so you can shop it along with 15a or 16.
   

Any one of these courses satisfy the intro mechanics requirement for the Phys, Chem/Phys, and Astro concentrations. The advantage of taking 16 or 19 is that you will learn about many advanced topics; see the course syllabi for a complete list of these. The advantage of taking 15a is that you are guaranteed to obtain a strong foundation in standard Newtonian physics. 

Note that Phys 151 (Advanced Classical Mechanics) goes well beyond the level of Phys 16, so if you eventually take Phys 151, it matters less whether you took 15a or 16/19.