That's going to be really difficult to answer here. It would probably be more effective to ask other students at your school who have taken those same classes from the same professors.
I do want to share some thoughts about the second part of your post:
12 credits isn't necessarily a ton, but depending on your limits and your willingness to push yourself you may or may not be able to handle more. What kind of balance are you willing to have in your life? The difficulty and work load of the coursework will be very school-specific, and everyone has a different level that they can handle. You are correct in pointing out that you will be expected to do quite a bit more work in med school and that you need to be able to show that you can handle it, but Rome also wasn't built in a day– you have to develop the discipline, study skills, and endurance to handle all of that material. You are the best judge of your limits. Over time, you will learn to do amazing things if you put in the effort.
To illustrate my point about the remarkable human ability to grow, here is my 2015 for you:
Winter/spring semester 2015:
Gen chem 2
Chem chem lab
Physics 1
Physics 1 lab
Molecular biology
Molecular biology lab
12 credits, 3.8 for the semester
Some volunteering, 15-20h/week work, intramural soccer team, no research, not much of a social life either
Summer 2015 (first half):
Ochem 1
Psychology
Started in a research lab
Started a volunteer leadership position
30-40h/week work, some soccer (but not a lot)
My grandma died right before one of my ochem tests and I had to reschedule the test to fly across the country for the funeral, which is a bit more difficult when the term is only 6.5 weeks long.
3.1-3.2 for the term (7 credits of schoolwork). My life sucked for these 7 weeks. I took on way too much, and I was involved in one of the above activities basically 6am-11pm Mon-Sat. I am fairly devout in my faith and do my best not to study or work on Sundays when possible.
Summer 2015 (second half)
2 credits of research
40h/wk work
New girlfriend
5h/wk volunteer
>3 hours of Halo/Skyrim/Ghost recon per day
Soccer with friends 2-3x wk
Life was glorious.
Fall 2015:
Ochem 2
Analytical chemistry + lab
Research for credit
Physics 2
Physics lab 2
Genetics
24h/wk work
Volunteer leadership
Captain of intramural soccer team
Significant progress in relationship with gf (wifed that later on)
I did a lot more research that semester than the credits I had signed up for– I designed a project and submitted a proposal for funding that semester, which was awarded the next year.
That was 15 credits total, and I ended up with a ~3.7 that semester. I was 3 test questions away from a ~3.9, and I could have gotten them right if I had known the material just a tiny bit better in 2 classes.
I just gave you a mini lecture not because I wanted to pull out my old AMCAS application and show much much I did that year (showing a 3.1 semester is not bragging), but to show that I was able to learn a lot about my own limits and really push them higher. That first semester I had 12 credits of fairly basic classes, not a ton of extracurriculars, and felt like I was rushing everywhere. During that first summer term, I bit off far more than I could chew, and I actually rushed everywhere–and my grades suffered for it. However, I also learned a lot about time management and my own abilities, and I developed a lot of important skills. Even though I wasn't perfect, I still managed to do well enough in that fall semester on top of everything that was going on outside of my classes. I took far more difficult coursework in the fall than I did in the spring, I took more of it, and I had more extracurriculars in addition to a more significant social life. I didn't just check the boxes in the ECs either, but I really ended up excelling in all of them (including the girlfriend!). At the time I wondered how I had felt so busy earlier that year with a far lesser load. I have since come to realize that I had learned how to be more effective with my time.
There is no one correct way to be a premed, and everyone has to work hard and be busy to get to medical school. I hope that by sharing my experience I can demonstrate that it is possible to learn to take on more challenges and that you can actually grow pretty quickly with the effort. I am learning how to learn all over again as an M1 now.
One final thing to add, however: 12 credits of 3.9-4.0 looks far better on your application than 17 credits of 2.9-3.0.
