If nutrient fertilization is needed, liquid is better for general purposes. I have maintained tanks with substrate-rooted plants (Echinodorus and Helanthium specifically) with just liquid and the plants were fine. However, adding substrate fertilizers (tabs, pellets) in addition to liquid will cause increased growth in swords and similar plants, as has been mentioned.
I have not experimented with using only the tabs, but I would suspect this might not be sufficient. Plants assimilate some nutrients from the water through their leaves, so these nutrients stuck in the substrate will not do much. I do have one tank with an enriched substrate which supposedly contains these nutrients. I do know that without liquid being added the same as in my other tanks, the plants in this tank did not do well; quite the opposite.
A complete liquid fertilizer, like the Seachem Flourish Comprehensive Supplement, provides all nutrients needed (except oxygen, carbon and hydrogen). There are a couple others that do the same. One of these ponce a week should suffice. But again, with the larger swords (like Echinodorus bleherae
, Echinodorus cordifolius
, etc) a root tab in addition will benefit.