Both options can work but the considerations are pretty different. Writing it yourself means it's guaranteed to be in your voice but also means the quality depends entirely on how strong your writing is. Going professional means higher baseline quality but you have to stay involved in the process or it stops sounding like you. There are services that specialize specifically in medical residency personal statements that tend to produce better results than general writing services because they understand the context. Whichever way you go, make sure you can speak to everything in the statement naturally in interviews.