They are complementary solutions and used for different cases. You can watch mobile TV programs when they are broadcast, but Web TV programs whenever you like. However, TV programs are usually available in the web TV service only for a limited time, and not all programs are necessarily viewable in the Web TV service. The mobile TV you can watch only where there is broadcast coverage while for watching Web TV you need to have an internet connection.
Web TV is based on internet streaming technologies while mobile TV (DVB-H, CMMB, MediaFlo, and ISDB-T) is based on broadcasting TV signal over dedicated networks. Instead of dedicated receiver hardware required for Mobile TV, Web TV requires an internet connection with WLAN or cellular 3G.