It depends on the kinds of plants you want. If you have rooting plants that need a good and firm substrate, the ultimate best substrate would be soil (safe for aquaria). Soil is best for growing healthy plants, combined with good lighting, and if you wish - with CO2 injections (but I personally wouldn't use CO2 injections because of the inherent danger it carries for fish).
Although I've heard that soil is not recommended for beginners due to several problems.
If this is just your average planted aquarium with cheap, low-light plants and no CO2 or anything, I wouldn't bother with anything other than regular aquarium gravel.
Yes, if you have low light plants, some appropriate gravel will suffice for rooting plants. And yes, soil is probably better for people who are already experienced with planted tanks. However, there's gotta be a first time. I think even beginners could be successful IF they did appropriate RESEARCH before setting up such a tank. Those who are serious about it, will resreach first. There are a lot of books and a lot of information out there on how to keep a successful planted tank with soil. Soil is the best natural rooting plant medium, especially for more demanding medium to high light plants.