I'd be sticking to sole trader as long as I could. I had to register a LTD company to get on the app store (otherwise your apps go under your personal name and address) and it's nothing but a headache. You have legal responsibilities as a director; you have to file annual confirmation statements; you have to send returns to *both* Companies House and HMRC - and forgetting to do any of this can get you into serious legal trouble.
Not to mention, when you've done *all* of that.. it's not your money in the bank account. Your LTD company is a legal entity of its own and every penny in and out has to be accounted for. You'll need to pay yourself either in dividends, a salary or a combination of both and then pay tax on those again via self assessment.
Trust me, as a side earner it's not worth it unless your arm is being twisted. The only thing I will say is if you plan to be doing bank transactions as a sole trader, you normally won't be able to name your account after the 'business name'. Instead of "Artco", your name that will show on statements will be "Steve Jones Trading as Artco" or something similar, and that's the name you'd have to give people if they were to send money to you too. Obviously this is all a moot point if you're taking payments over stripe etc.
P.S you can have as many sole trader businesses as you like! The self assessment lets you create a business and add the profits / expenses to each one individually.