We would advise you to consider COMP1917 as you will learn more and be stretched.I'm doing Adv Maths majoring in Applied Maths. I'm going to be taking Mathematical Computing this semester and Computing for Finance in 3rd/4th year. Would you recommend I do COMP1911 or 1917?
BUT - you need to consider the amount of time that you need to to devote to the core component of your degree and be aware that COMP1917 is intensive - student who don't need to take COMP1917 should be careful not to disadvantage their core study by taking an intensive computing course.
Assuming that you are keen for a challenge, you should enrol in COMP1917 and then, if it is too much, drop down to COMP1911 BEFORE the 31st of March. It is much more difficult to start in 1911 and then go to 1917 but dropping down is relatively straightforward as long as it's done BEFORE the 31st of March.