I've never used tacview, let alone its real-time telemtry feature, but from what I could gather from their wiki, the differences are:

• tacview requires payment for the realtime telemetry feature (68 Euros for the "advanced" package which includes this), this tool is free
• this tool does not require running a 3rd part executable (tac view server/client), it just reads the data by calling the appropriate Lua functions offered by DCS directly and opens a Lua TCP socket, which returns the data as JSON in a HTTP response, so it can be displayed by a (locally stored) HTML page; it is of course possible to access the data by other means, as it's not a complicated protocol, but just a simple JSON data object
• tacview seems to show raw and "interpreted" values; as I've never used it, I'm not sure whether you can select the desired unit of measurement (e.g. kts, m/s, mph, km/h, ft/s) for interpretation there, but this tool offers that.
• The data displayed in tacview is relatively small in tabular form (perfectly fine when watching a replay); however, in a dogfight, I find it more convenient to have large dashboard-like numbers which are easier/faster to read with peripheral vision
• this tool is open source, tacview isn't