TFC's value is higher because it has more assets that are available to a potential buyer. TFC owns its stadium. Buildings are considered significant when assigning value as they can be used for more events.
Perhaps more importantly, buying TFC is essentially buying into Major League Soccer given the single entity ownership. As the league profits so do each of the investors (franchise operators). Buying into MLS means shares of expansion fees, league revenue, shared revenue from other clubs and perhaps most importantly a share of Soccer United Marketing (SUM). This is the entity that has held World Cup TV rights, rights for the USNT, rights for Mexican National Team games played in the US, the USSF, etc. It is VERY profitable.
MLSE could do nothing for TFC and it could continue to profit from the dividends that the league generates. They don't have invest another nickel (unless the MLS requires them to do so) and they continue to make good returns. Their attendance could fall off the map and provided MLS does not revoke their "shares", they would continue to make good returns.
The Argos on the other hand represent an interesting opportunity for return on an investment. The Argos have no stadium but MLSE does. Buying them immediately nearly doubles their gate driven rental income... generating as much or potentially more than TFC based on attendance figures and playoff dates... and allows them to reach a much more lucrative TV contract. All for the investment of Jerman Defoe's rumoured total contract value. Given the revenue TFC is said to generate which is a combination of gate receipts, TV/advertising and merchandise, it would seem that owning the building would enable MLSE to pay off the Argos investment within the first year(s). And they don't have to share any of the profits with anyone else.