What is the best storing device for DB transaction log?
You really need a fast HDD, but you need a properly sized innodb_log_file_size. 

Why not SSD for MySQL ? I learned something from this layout from a FaceBook Engineer's blog

I wrote old posts about this


Aug 14, 2013 : How do I determine how much data is being written per day through insert, update and delete operations?
Feb 06, 2014 : MySQL on SSD - what are the disadvantages?
Aug 12, 2014 : Wordpress blog where pages are almost-always served from cache: SSD or more RAM?


I even mentioned doing this for PostgreSQL : Postgres Write Performance on Intel S3700 SSD

BTW See my post MySQL 5.5 - Innodb - innodb_log_file_size higher than 4GB combined? for determining the right size for innodb_log_file_size. You don't want a small HDD for ib_logfile0 and ib_log_file1.

Give it a Try !!!