You should definitely do some research before you apply. There's a very strong likelihood you'll get biased answers on here. Both the Army and RAF Recruitment websites give you a lot of detail and only YOU can define what's important.
Some things to consider though (as unbiased as possible).
- As an officer, there will be an element of Administration, but as a junior officer, that's less so.
- It sounds silly, but the two roles you've mentioned, although the same in title, will be largely different in day-to-day work and responsibilities as well. You'll have a lot more front-line contact with the Army, whereas the RAF tend to work behind the wire, or in the Air. This would be one of the thoughts that I consider.
- As well as that, look at the training for both services in detail... RAF Initial Officer Training is 24 weeks, whereas the Commissioning Course at Sandhurst is longer and arguable more intense. There's also the very small detail that you join the RAF with a job role, whereas unless you're joining the Army with a guaranteed role, you'll join as an officer and you'll need to interview during the commissioning course for the role you want.
Lastly, I'd advise watching the 3 part documentary on Sandhurst (just type Sandhurst into YouTube and you'll see the ones I mean). Even though I'm joining the RAF, I still really enjoy watching it and although they were filmed back when Afghanistan was the main talking point, you'll get a feel for what it means/takes to be an officer.
I hope this helps.