I have the sennheiser hd 449 and they are great closed cans with surprisingly good soundstage for a closed can but you say you like punchy bass and 50% of your time will be one gaming so , i would recommend the hd 518. why? :
well the hd 518 has punchier and stronger bass
it has better soundstage (because its an open headphone)
note that since its open it leaks sound so if you plan to bring these on the road , people will hear your music
the hd 518 are more comfortable and wont make your ears sweat
the hd 518 velour pads pick up dust , so if you live in a dusty house , hide them from it
the hd 449 has better mids(female voices,snares etc)
it uses pleater(fake leather) and your ears will heat up depending on the temperature of the day
the pleather pad are easier to clean
the hd 449 are ideal for traveling because 1. they are lightweight and easy to carry around and 2. they block outside noise and dont leak sound (unless you crank up the volume to very high levels)
and they look cool