6m X axis Rack and pinion or belt drive?

[JAZZCNC]

That does look very interesting however my fear would be it would be prone to getting clogged up with foam dust under the belt. I would also like it to be substantial enough to use for cutting wood which i assume the belt should be all be it you would have to cut slower.

Well that depends where you put the belt and how you shield it plus you'll have the same same problem with R&P regards keeping foam out of the Rack. Belt drive will easily cut wood and it's also much better regards efficiency and accuracy, much less backlash than R&P.

ignoring cost and setting up what are the disadvantages of rack and pinion on something this long?

Not too many really other than it's less efficient and accurate than belt drive. In all honesty I'd personally use R&P has it's more proven and will easily cover your needs but you need to be aware of what's involved with R&P to get best from it.
One of things you need to be aware of with R&P is that it's less efficient with much more friction so will need more powerful motors/drives unless gantry is light weight.
Regards the gantry you say your having a cantilever design and would like possibility to cut wood.? Well unless you substantially build the gantry then you will struggle to cut wood has it will deflect at the outer ends. This will definitely take you into Nema 34 size motors which require larger drives with high voltage so bigger PSU to get the best from.

This leads to the next issue with R&P (And Belt drive.!!).? . . Resolution, Torque and Direct drive. The pinion size determines Resolution and even the smallest pinion size still gives a resolution that is too large to be useful. IE: If you use common Mod1 rack the smallest pinion is 12Teeth and 12mm DP so the affective pitch would be 12xPi= 37.7mm so your pitch would be 37mm which is quite large but nearly usable.!! . . .But it gets worse.??

12T pinion dimensions are too small and won't fit on the motor shafts so in reality you'll end up around 24-25T but now the DP is also 25 so 25xPi=78.5mm affective pitch.!! . . . . Massive and useless for anything other than linear speed.
Also on top of this you have friction to over come and R&P is quite inefficient at around 50-60%, compared to 80-90% for belt and 90-95% for ballscrews so this is why larger motors are required but with direct drive they would need to be larger still.
So to help reduce the motor size/load along with increasing resolution then you need to have a ratio applied, what ratio will depend on several factors.? Resolution and feeds required, Weight to move, motors used, sticktion to overcome.!! etc
Often 3:1 ratio is used which at 25DP is still around 25mm pitch which will give high feed rates around 20Mtr/min and given an average micro stepping of 800Ms give theoretical resolution of 0.03mm, say theoretical because you won't actually get that but even if it was double still ok.
You mostly need the ratio to give the torque and get the Resolution down to a level that the motors work best with.? One of the problems with Low resolution is the speed, the motors struggle and are very unhappy when low feed rates are required has they are hardly spinning so can give very rough action. The ratio needs to be a balance of providing the best torque and resolution while still keeping the motors spinning in speed range they like at lower feed rates.

So has you see it's not just simple has R&P or Belt drive, which by the way you still need a ratio for belt drive has most of the above applies with the exception it's more efficient so less Sticktion to overcome and can often use smaller Nema23 motors.

Hope this helps rather than blows your mind. . .Lol