rlly depends on your skillset, physics is obviously a science so u need to be able to explain scientific theories and all that, whereas in software you're more developing solutions to case studies and building systems to solve problems. for both subjects you need a grounding in basic algebra/math, not much past that, however i will say that software does probably use more maths since you'll be doing algorithms which are pretty grounded in logic and usually some basic math, although don't stress over the math in either (i'd recommend taking advnaced math though). for both subjects you're also spending pretty much half of the time solving problems (in software usually building a system or new algorithm to solve a problem, in physics you'll be doing more "computations" although setting up the problem is still a big part of it) , half of the time on theory - in software you'll be writing about solving case studies and why certain things you learn about work, in physics you'll usually be explaining how a result came to be (experiments) or how a certain device works.
for me software was much easier because i found the topics of writing to be easier and i also had a small background in programming, nothing crazy but still i found that i absorbed the concepts pretty easily. on the other hand i didn't really have a good scientific background going into physics (didn't even know what a "srp" was smh and i picked 3 sciences what was i thinking) and it took a while to get used to the writing style, physics markers at my school were also very strict although valid, and set fairly hard exams so there was definetly a learning curve for me; although it worked out and i eventually got a band 6 after a lot of failure. however i also know people on the other side who found sdd hard but physics easier, as i said it's really just a matter of your background and your strengths
on a side note the new syllabus for sdd/ipt is so good and im so jealous like fr it's so new and more interesting concepts instead of our 2008 syllabus ugh i'd really recommend taking both if you're interested