With the words "right now" included in the OP, I would actually take Tavares.
Both are paid as top-line guys but neither are the #1 C on their team (Lindholm in Calgary, Matthews in Toronto). However, despite being older, Tavares is still scoring at a higher rate (currently around #50 in the league) and has done so throughout his career. Even as he ages, I'd argue that Tavares' contribution might not drop too much since his game isn't very speed-oriented, such that if they convert him to the wing later he could still make an impact at a high level.
Of course Monahan may prove this "slump" (and with all due respect 0.69 points/ game is not really a slump) is just momentary and come back hot, but there's still no certainty that he could reach the same offensive heights and consistency that Tavares has, so unless he really slows down in his scoring I'd say John's is still worth it as a top-flight player in the NHL, therefore warranting the contract.