There are 3 kinds of Express buses, Premium Gold, Premium and Regular Bus.
Price and Comfort: Premium Gold > Premium > Regular
Time to travel: Premium Gold & Premium are using Express High Way most of cases. Rarely National Road. Regular Bus also use Express High Way but some times, it takes National roads which takes more than the express high way. Therefore, when you buy a ticket of regular bus, better to check if the bus take fast way or national road.
From Seoul(Dong Seoul Express Bus Terminal) to Sokcho, there are only 2 options.
Premium Bus or Regular Bus.
Premium Bus takes high way, fast, and 15 min break on the way at the rest service area where you can enjoy local culture.
Regular Bus takes national road number 44. It takes 40~50 min more than Premium bus, no visiting service area, only Wontong local Terminal you have 10 min break for rest rooms. Then it visits every local small village in Gangwondo region, including Hangyeryeong. That summit is the one of the highest and most beautiful ridge in East parts in Korea. Unfortunately you won’t be able to take a break here, but you can enjoy the sublime nature while the regular bus drives to Sokcho.
What I recommend, if you have chance to go Sokcho from Seoul, try to regular bus for one way and return by premium bus, it would be good to enjoy both journey. The price difference is less than 4.50 USD. It worth to take regular bus even if it is less comfortable(4.50 USD cheaper), to enjoy the Seorak Mountain area.