1 If you (1)have strong record; (2)good at English or have American friend can help you on grammar check, and (3) you have resource to collect sample petition and recommendation letters, do it yourself!
2 if you hire a lawyer, don’t rely on them! You have to do your own homework by looking through related website and learn from others experience !!!
I hired a attorney, but very disappointed with them, they didn’t prepare a persuasive petition letter with my original submission. After receiving RFE, I took that they would help a lot on RFE, but I were wrong again, they didn’t even pinpoint my advantages in their RFE cover letter (they don't have time, too many cases). So I almost prepare RFE by myself based on the experiences posted by name-unknown nice people on this website.
3 Petition letter is very very important!! Citing recommendation sentences from reference letter is not enough from my experience, officer wants to see the good words from published paper written by others, more convincing!!
4 only using strong words to sell yourself, avoid week words. length isn't important.
Good Luck !!!