matter of opinion, a
A question on which people hold different views, as in I rather like that design, but really, it's a matter of opinion, or The quality of that new stock issue is a matter of opinion. This expression was first recorded in 1843.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.