What is the difference between 会 and 说 in 你会英文吗？ vs. 你说英文吗？
This is exactly what I stuck in at the very early stage of learning Mandarin!

会 is usually used as an auxiliary verb (mean "can", "will", etc...) and thus take a verb after that, but it can also function as a verb, to mean "know" or "understand".

That's why in your example, both mean "Do you speak English?". 

Also, if you want to use 会 as an auxiliary verb, you can also say "你会说英语吗？".