You can do it either way, you'll still get the same answer. It depends on which side you like your m to be on, or which you would rather be negative.
2m-8 = 5m -7 --> -3m = 1 --> m= -(1/3) *This method (+8) gives me an m on the left side but makes the m coefficient negative.
2m-8 = 5m-7 --> -1 = 3m --> -(1/3) = m *This method (+7) keeps our m coefficient positive.
I personally like keeping my m-coefficient positive, but whatever you prefer or works better for you is fine.