Given your weak background in Spanish, it sounds like the IB Spanish Subsidiary Level (SL) class (which IS a two-year course) would be the best fit for you. You may find that both AP Spanish and IB Spanish Higher Level (HL) are too rigorous and demand better Spanish language skills than you’ve acquired so far.
Here is a Web site with some information about the differences between IB HL and SL Spanish. http://en.wikibooks.org/wiki/IB_Spanish#Differences_between_HL_and_SL(Note that colleges often do not give credit for SL level classes; most will give it for high scores on AP or HL exams, although policies vary greatly.)
I suggest approaching the head of the Spanish department at your high school (or any good Spanish teacher at your school … not the snoozy señor ;-)) to ask for advice. These teachers should have a sense of the rigor of each class as it is taught in your own high school. As you have probably figured out, the difficulty of a high school class can depend not only on the curriculum but also on the teacher. Some teachers have higher expectations than others, and their classes can be far more demanding than a class with the exact same title that’s taught by an “easier” teacher. So that’s why it’s important to ask a teacher at your own school to help you with your choice.
If you’re worried that any Spanish class you take might be too challenging for you because of your weak foundation, check out local programs or online courses that you can pursue over the summer to boost your skills before next September. You don’t necessarily have to take a formal class. You might find a program in your community–or a free Web site–that will allow you to practice your Spanish without feeling like you’re doing school work during your vacation.