I for a fact know that planted tanks are cleaner, as plants abosorb bad gases inside air bubbles hidden in the sand, and they also reduce heavy metals and produce oxygen for your fish. I use at leat 1.5 inches of sand for my planted aquarium, but some people like to create little hills and stuff that makes it look real cool. I must warn you though, if you want to heavily plant your tank, you must be ready to provide CO2 and alot of light... C02 systems can be bought or made, and simply google or contact your LFS for any plant friendly lights. Good Luck
Plants absorb bad gases inside air bubbles? There are the fears that hydrogen sulphide can develop in sand, but having snails or just turning the tank over prevents that, and I for one have never heard of a first hand account of a fish dieing from it. I would also be interested to see any evidence of plants taking out heavy metals (something a number of water conditioners handle anyway).
Most plants on the whole do NOT produce a lot of oxygen for the fish. Come night time, with the lights off, plants respire - they consume oxygen and produce CO2, leaving less oxygen at night for the fish. On top of this you have the problem of not wanting any decent gas exchange at the surface to prevent the loss of any CO2, thus preventing a good source of oxygenation.
Basically, with planted tanks:
PRO:
Great to look at if you like gardens or the colour green;
Great environment for smaller fish to cruise around in and you only keep smaller fish.
CON:
No big fish. Big fish need lots of oxygen which means lots of surface movement, which is bad for plants;
No super filtration such as wet/dry or sumps as again, these mean more gas exchange;
No super sensitive fish. All those places for waste to accumulate and break down where you can't get it out can play havoc with a system for sensitive fish.
If you like the look of a planted tank, go for it. If you would rather keep fish, then don't bother, or at the most put in a few hardy ones that aren't fussy (crypts and the like).