A simple way to split Swing and Rockabilly is Pre- and Post- WW2. Swing c1930-1945 and Rockabilly c1945-1960. However, the current trend of pop 'vintage' now blurs these lines.
Artists such as Brian Setzer and Imelda May mix the musical styles. As the music has shifted and begun to merge, so has the distinction in fashions and culture. However, while there has been some convergence between Swing and Rockabilly, there remains a clear style and appreciation for each. As a generalisation, Rockabilly has more edge and more attitude. Where Swing tends towards the elegant and refined, Rockabilly is raw power and passion. Where Swing elaborates in softer accents and complex rhythms, Rockabilly is driven by thick, soulful blues and strong, heart-pounding beats. Rockabilly was the hard rock of its time and the rebellious streak remains.