Do they mean exactly the same thing?
(half-decent and halfway decent...)
Spend the extra money to stay somewhere half-decent?
Spend the extra money to stay somewhere halfway decent?
Are these adj words positive or negative?
As far as I understand both "half decent" and "half way decent" have exactly the same meaning. They are both slightly positive in meaning. For example a "half decent hotel" would maybe be a 3star hotel, not top of the range but not the worst either.