As a former club dj and as current employee for one of the worlds largest aluminum companies here is my two cents worth.
First regarding DJs and direct drive tables
This is not true unless you are a bad dj and don't know how to mix. DJ's are attracted to the SL1200 for its ability to accurately adjust and maintain a steady speed with the pitch adjust. As a dj you want to mix the music such that the BPM's of the next track are in synch with the one playing so the change from one to the next does not disrupt the dancing of the crowd.
Second, upon purchase of a new table by a dj, the rubber mat that is provided is thrown out along with the rest of the useless packaging, then a felt mat is used to allow the platter to spin beneath the record such that the racord can be held in place or pulled back during the cueing process without disrupting the movement of the platter (it act as sort of a coupling). If you are scratch DJ you may even chose to add teflon record sleeve cut into a round shape between the felt and platter just to be sure there is no sticking.
Now that this is cleared up, high inertia could be benefitial for DJs provided the performance of the slip mat "felt coupling" can prevent the fly-wheel from slowing. So far the direct-drive quartz speed adjust is more reliable.
Aluminum or Aluminium, not much to add other then to confirm what has already been mentioned, it is relatively inexpensive, non-magnetic, easy to machine, cast, or forge and readily available in wide variety of sizes and formats. Copper has become extremely expensive. Steel is cheap but difficult to machine, magnetic and will it rust unless protected, bronze contains copper so it is also relatively expensive. Finally lead, is toxic. There also magnesium, berylium and titanium, these are expensive, difficult to machine (specialy for Mg, which the chips are flammable and potentially explosive) and toxic in the case of beryllium. So in terms of metals Al is pretty easy choice.
As for other materials such a plastic, carbon fiber or wood, I can't comment, but I would image they could be good choices for the given application.
First regarding DJs and direct drive tables
when you are a club dj you want to be able to cue up the record and have it accelerate to speed as quickly as possible on the break.
This is not true unless you are a bad dj and don't know how to mix. DJ's are attracted to the SL1200 for its ability to accurately adjust and maintain a steady speed with the pitch adjust. As a dj you want to mix the music such that the BPM's of the next track are in synch with the one playing so the change from one to the next does not disrupt the dancing of the crowd.
Second, upon purchase of a new table by a dj, the rubber mat that is provided is thrown out along with the rest of the useless packaging, then a felt mat is used to allow the platter to spin beneath the record such that the racord can be held in place or pulled back during the cueing process without disrupting the movement of the platter (it act as sort of a coupling). If you are scratch DJ you may even chose to add teflon record sleeve cut into a round shape between the felt and platter just to be sure there is no sticking.
Now that this is cleared up, high inertia could be benefitial for DJs provided the performance of the slip mat "felt coupling" can prevent the fly-wheel from slowing. So far the direct-drive quartz speed adjust is more reliable.
Aluminum or Aluminium, not much to add other then to confirm what has already been mentioned, it is relatively inexpensive, non-magnetic, easy to machine, cast, or forge and readily available in wide variety of sizes and formats. Copper has become extremely expensive. Steel is cheap but difficult to machine, magnetic and will it rust unless protected, bronze contains copper so it is also relatively expensive. Finally lead, is toxic. There also magnesium, berylium and titanium, these are expensive, difficult to machine (specialy for Mg, which the chips are flammable and potentially explosive) and toxic in the case of beryllium. So in terms of metals Al is pretty easy choice.
As for other materials such a plastic, carbon fiber or wood, I can't comment, but I would image they could be good choices for the given application.