Depends on what species of plants you are keeping. Some plants (like swords and crypts) are heavy root-feeders, and would rather have root tabs. Other plants are water column feeders, and benefit from ferts in the water.
Kent is ok, for starting out with. Don't waste your money on Leaf Zone, Tetra Flora Pride, and anything from Mark Wiess.
People that are really into plants (myself included), tend to use dry powdered fertilizers. These can be found here: http://bestaquariumregulator.com/ferts.html
It takes some testing (particularly nitrate, phos, pH, KH) and some more knowledge before you'd want to jump in that quick though.