Either sand or bare bottom. For cleaning and feeding purposes, a bare bottom is the very best choice, but many people find it rather unaesthetic. If you use sand, make sure to have a layer no deeper than an inch, or else anaerobic areas may form.
I am an avid supporter of the bare bottom tank. Bare does not have to be truly bare though. I am a huge supporter of photographic bottoms.
This is easy to do with the advent of digital cameras and cheap hi res printers. I take pictures of assorted surfaces (usually rock) and then print it out in full color on some photo paper I get cheap at Costco.
Then I merely cut and paste the photo to the exterior bottom of the tank, picture side facing to the interior. Other times I do multiple cut outs from multiple pictures and collage them.
The result is easy, instant custom tank bottoms with all the beauty and none of the troublesome issues with real substrates!
But, Like Midnight stated, there are a ton of threads on the subject and a wealth of data on Caudata Culture and our other sister sites. A bit of effort with the forum search function is definitely advisable here.
the bottom line is, substrate bottoms make it easier for live food (e.g. blackworms) to hide and for uneaten food to go unnoticed and spoil the water. Bare bottom tanks make cleaning up much easier.
Alternatively, you can even have a planted tank with a soil-based substrate (search for "walstad method" or walstad tank), which is very pretty, but requires some attention to detil and careful water monitoring while the tank cycles (which is a good idea for any newly set up tank).
If you go bare bottom, you can still have live plants if you either use rootless plants or those that grow floating (e.g. hornwort) or plant your plants in shallow dishes which you set into the tank.
i highly recommend “buyanaxolotl.com”- ive purchased from them and received a beautiful animal for relatively cheap, in great condition, and excellent shipping precautions. the breeders are a couple living in georgia (i believe, don’t quite remember) and they’re fantastic. sometimes their website contact page doesn’t work, so i’d probably try just emailing them. good luck and happy hunting!