It's best to take more credits and get high grades, but that isn't an option you listed. Why are you supposing that more credits will equate to you earning lower grades? 12 credits is probably full-time at your school, but you should pile another class on top of that to bring your total to at least 15-16 credits each semester. Having a heavier courseload will demonstrate your maturity and ability to handle such a load. Of course, don't pile on the courses until you drown yourself, but challenge yourself. Nearly anybody can do well in science courses if they only have one -- the true test comes when you are taking biochemistry, pathophysiology and other classes. This semester, I have:
Honors Lit. 3
Bible as Lit. 3
Physics & Lab 4
TA Physiology 1
That totals 21 credit hours. I certainly don't recommend this (particularly if you aren't accustomed to the college experience), but I've found that this number of credits keeps me motivated, challenges me, and that I'm still able to get above a 3.5 every semester with this number of credits. I also work 15 hours a week, am married, and have a seven-month old son. It isn't impossible, and it keeps me focused most of the time.
When I was a sophomore/junior, I was equally busy. I carried 17-22 credits through each of those four semesters (lowest semester GPA was a 3.48), and worked 40 hours a week for those two years. Once again, not easy, but fairly doable. Anything less and I would be bored!
Just take as much credits you think you can handle. If your plan to take mainly all difficult science classes than maybe you should stick w/ 12 or 13 b/c I know that the quarter system goes really fast. Those 10 weeks go by like nothing. People who's on a semester schedule don't really understand how fast a quarter is. It bugs me when my bro always nag me about only taking 12 units cuz he just doesn't know what its like. So if your taking alot of difficult classes I recommend not taking 16 credits especially if you have other duties like volunteering and stuff.
If I have the choice of 1) taking way too many classes and end up doing badly, and 2) taking too few classes but do really well in them, #2 is definitely the more conservative, "safe" route. Problem is if you do #2, you might not get your Bachelors in the standard 4 years and that might raise some eyebrows.
Those are extremes though. You should shoot for something in between. Find a balance-- Take just enough classes to maximize your capabilities.