Art IS very time-consuming, much more time-consuming than Geography, but also keep in mind that this time-consumption tends to only be a problem with people who aren't interested in pursuing Art; Any art you do outside of school can still be put into your coursework under the guise of "experiments" and "practice", so you can gain marks for engaging in the hobby regardless.
Art will take up much more of your time, but I personally felt like learning more about the visual arts was very useful (As a complete STEM kid who didnt care much for Art before). I'd also note that so long as you're taking a humanity at all, you should be fine. Though this isn't a universal rule (And you SHOULD check with sixth forms you'd consider later on about this!), whatever GCSE subjects you take will usually be fine. I was accepted unconditionally into Sixth forms where I hadn't taken a single subject I chose at GCSE, with things like RE and Philosophy (Of which I did neither.)
This isn't a hard rule, and some sixth forms are stricter about this, but generally Sixth Forms will be more lenient on GCSEs because you're more likely to make irreversible choices which you regret. If you make up the grades in History, and you make the case that you now want to do Geography (and you know what the risk is of taking a subject you didn't at GCSE), most sixth forms will be more than willing to accept you and moreso if you're applying to the school where you did GCSE's.
Basically, just choose what you enjoy more. If you're taking a humanity, you'll most likely be fine regardless. Always check first though!!