There are pros and cons of taking H1 or H2 Maths:
H1 Maths is an easier version, and will definitely take less time to study. This time can be used for studying other subjects. Also, it covers statistics which can come in handy for majors like Psychology, Social Science, or Business. Students who take A Maths in O Level will find that the Pure Math part of H1 Maths is basically the same, if not even easier than O level A Maths.
H2 Maths is the harder version, more difficult than even the O Level A Maths. New and interesting topics like Complex Numbers and Vectors will lay a good foundation for University majors like Engineering and Physics. Try not to forget what you have learnt in O Level A Maths, it will come in handy.
Students who wish to enter SMU & take H1 Maths in JC may want to note that SMU has a introductory module on calculus which is pretty much compulsory, even for majors like social science. I have taught a student from SMU, and would say that the content is heavier than even H2 Math Calculus; there is multivariable calculus in the SMU Course.