It really depends on your ability in maths and also what course you desire to enter in university.
Maths Extension 1 scales rewardingly and so can provide an ATAR bump if you do well in it. However, I would advise against doing Maths Extension 1 only for the scaling if you're not comfortable with your mathematics ability.
If you aren't comfortable with your ability in maths or feel like Maths Extension 1 might be too difficult, then it will probably be better just dropping it and sticking with normal 2U Mathematics and acing that one as it will relieve your study load and allow your more time to focus on your other more stronger subjects. This could also be beneficial ATAR-wise.
Also, perhaps consider what University course you want to do later on. If you're interested in Engineering or a Math-related major in Science, it may be useful to take Maths Extension 1. However, if you don't feel comfortable with doing Maths Ext 1 and you are interested in Engineering or something similar, you can always do a Maths Extension 1 Bridging Course during the summer holidays after HSC.
Overall, I would just do what you think will make you most comfortable and be most motivated.
Hope that helped!