Which one is better?
I need to store a lot of static files ... and I wonder which one is better, cheaper and faster.
It may be helpful if you can give a bit more info about your environment.
EBS will probably be faster to a point. They are essentially attached storage. S3 is meant to store a lot of static files and has good download throughput. However, S3 is "eventually consistent" which means that your data may not immediately appear, so if you are writing a large number of static files to be read somewhere else soon after, S3 may not be the best option.
Common use cases for S3 include static content storage like website images, downloadable files etc.
For EBS over NFS it should behave as any remote drive, but be prepared for any trouble if you are reading the NFS mount over a large network gap (such as outside of AWS to inside of it), then S3 may be preferable.
EBS costs $.10 per GB-month (so $1 per 10GB each month) - EBS Snapshots (basically a backup copy) are stored in S3 and charged at the S3 storage rate.
S3 costs $.15 per GB-month
Also, don't forget about I/O and data transfer costs for each service, as these vary based on if they are internal or external.