It’s up to you man. Things to consider are:
1) You have a job that might get pissy about it? Inner is easier to conceal than outer.
2)You have a career in mind or any job aspirations? Don’t let ink sabotage your dreams for the future. Again inner is easier to conceal.
3)Do you have any other plans for inner or outer forearm work? I find myself regretting smaller pieces on bigger canvases just because I now imagine something much bigger there which just won’t look right with what I’ve already got or won’t fit because of it.
4)Do you want to be able to see it all the time? Unless you’re dead bendy forearm might be a bit difficult to see youself lol

Sorry I didn’t give you a ‘this or that’ answer but hope those things to think about help you decide 🙂