I use a deep bioactive sand/soil mix. Before that I used PVC shelf liner and before that tile. You can be quite successful with a range of substrates, these are hardy animals. Of course some are better than other for particular reasons. I don't like plain sand (and very much dislike artificial materials) - they don't really offer enough benefits to outweigh the work they require or the risks they have. Mostly I talk about tile and soil because those are the two I like best and tend to reccomend out of personal preference.
When deciding a substrate I take a few things into consideration.
Safety - different substrates carry different levels of risk (either from clumping, sharp edges, dust, bacterial growth. Etc). What is the best way to give the animal an environment it understands - most dragons don't come from areas with loose shifting sand dunes but rather savanna or forest edge sort of environments.
Experience - it's easier to learn when you have fewer variables. Husbandry become more important with loose substrate. A complex enclosure will require more attention and understanding of both the dragon and the setup.
Animals health - a compromised or ill critter may require special consideration for substrate and overall layout. Dehydration and insufficient heat are also part of the risk for impaction, animals suffering chronic dehydration or with kidney disease may not be able to pass the same particles that a healthy dragon would. a healthy
hydrated dragon shouldn't have problems but an undersipported dragon may seek out soil to eat for minerals or become impacted firm dehydration.
How much work you want to do with the enclosure - tile and solid substrates (and plain sand) are on a "sterile" principle. To keep such and enclosure a healthy and safe environment you need to clean frequently. Tiles are pretty quick to clean, especially of you have ones you can just swap out. As mentioned sand tends to be more difficult to make sure you have all the waste out, and needs to be completly replaced periodically. Bioactive soil requires a bit of care, (feeding and watering, so to speak) it's more like a fish tank in this regard. I don't like carpet, it just seems really gross.
How you want to offer furnishings and enrichment - Tile doesn't allow the animals to dig, for example but you can give a dig box. You can still get creative with tiles and such and build enrichment in with different levels, caves, branches etc. I like soil because it holds burrows well (dry sand doesn't), and I can achieve a more natural experience for my animals. Of course you need to have a bit of love for bugs and creepy crawlers for this
Enclosure size - tiles can be cut to size pretty easily and quite small space wise. a bioactive soil should be about 6-8" deep minimum to work well, this takes up enclosure space and is heavy.
Personally, I love the soil. I have my dragon, both snakes and my feeders on it now. There is a great Facebook group (reptile and amphibian bioactive setups) that's worth checking out if you want to learn more.
