id say that if you are planning on putting bottom dwellers in the tank then you would want a sub that isnt sharp in any way.
also, many people prefer a dark/black sub because it shows plants and fish nicely...good contrast.
its nice to have nutrients in your sub, this will help your plants tremendously.
like i said in your general thread, ada aquasoil is probably the best. i use amazonia and love it. my new tank is blossoming with this sub so i recommend it, as do many other people that have used it.
one thing i really like about the amazonia is that it puts lots of ammonia in the water column and my plants love it. it also softens my water and lowers the ph. that is important to me because it creates good water parameters for my plants and certain fish that i want to keep that like/need those water conditions.
there are many others that will work, including an inert gravel. however, more effort on your part may be necessary to get the plants the nutrients that they need.