Hey there! I'm in Year 11 doing both at GCSE, so hopefully I'm qualified to answer this!

Though course content will differ depending on what exam board you're with for them ( I'm with OCR in Computing, and Edexcel in Geography), I'm pretty sure difficulty doesn't vary too wildly.Geography prioritises case study knowledge - learn 5 points of data for each case study, apply these in 8 markers, you're set to get full marks in that question. Some small questions are relatively easy if you have common sense - e.g., how has agriculture lead to climate change (3 marks). Hint: mention fossil fuels and greenhouse gas emissions and you're fine!
The main issue in Geography I think is the dullness of the course content. You seem pretty interested which is awesome, and your specific course may have slightly more interesting content. However, the 'human' content is what I've only found to be interesting, and the 'physical' and 'fieldwork' section is gruelling not in terms of difficulty, but in how non interesting they are. If you can revise and remember processes and data (judging by your expected grades I think you can) then it should be no problem for you, as unlike other GCSE's the questions remain very very similar and easy to answer, for the most part.
Computing is slightly more difficult, but not enough to scare you! The hardware stuff can take a while to get lodged in your brain, but if you revise well and often it is a doddle. With OCR I like that I don't do programming coursework as a marked piece - this I found quite dull weirdly enough not because of the programming, but you have to explain your every line, and it takes 20 hours to do! However, unlike the 1st paper in hardware the 2nd paper is to do with programming, not only how to do it but the theory behind it as well. This has a lot less 'heavy' content to memorise and more just grasping how to program - again, with dedication you will learn.
Right... so usefulness. For this I would have to go Geography. Computing is very well respected ( this doesn't go for ICT I think ) but unless you're thinking about taking it A Level / beyond it won't offer you much specific benefits. Geography will open up more routes at college typically -
many people want to do a humanity at A Level and a level 6 is required in a humanity usually. Also, a lot of A Levels require a 5 at least in a written subject along with English, and Geography makes a nice candidate.
So overall? Up to you. Geography is helping me choose my A Levels by opening a lot of doors, easy questions, but I've hated the boredom of the content and remembering data is a bit of a pain. CS has quite a bit more to learn, but I've enjoyed my time with it and I'm going to use what I've learnt in my personal Python side projects to boost my CV, even though I don't think I'm going to go down that path.
I hope this was helpful! Feel free to ask about anything.