Mk 9s are obviously bigger and louder, but consider why you want the loudest.
Some sites will be restricted on even having things like a Mk 9 (e.g. council restrictions on their licence due to residential areas etc) and people may not be so impressed with you after they regain their hearing from a Mk9
Though these have a lesser level then mk5s, they have a reputation among the recipients of quite a bang and flash - possibly because they are often used indoors, but also because they act differently to a standard flashbang.
http://www.enolagaye.com/thunderflash/flash-grenade/
Bearing in mind that camera microphones tend to have some protection and restrict volume the sound comparison is not true, these give a rough guide to the difference between a Mk5 & Mk9