The main difference is about Arminianism v. Calvinism. John Wesley was of the Arminian school of thought of the Church of England so all denominations descended through him (Methodists, Free Methodists, Nazarenes, Holiness Pentecostal, Salvation Army) tend to also hold to this view point.
Methodism (I can't speak on behalf of the other Wesleyan groups but I reckon they'd be the same) holds to the traditional view that an individual can only be baptised by water once in their life, and so has confirmation for those who want to make a public declaration of faith who were baptised as infants. Under no circumstances will a Methodist church re-baptise someone. Baptism can be by total immersion or it can be by sprinkling someone with water, or by dipping the person in water.
Baptists only baptise adults, and only baptise by total immersion. As far as I'm aware, they are prepared to baptise an adult even if they were baptised as a child in a different denomination (some baptise you again even if you were baptised as an adult), and so don't hold to the view that you can only be baptised once.