There is no similarity at all between the Philidor and the Tennison Gambit.
First off, one of them is a Gambit played by White and the other is a Defense to 1.e4 by Black
One is sound, but passive. The other is completely unsound!
Philidor's Defense - It is sound, but very passive compared to other defenses, such as defending the Ruy Lopez, the Petroff, the Sicilian, the French, or the Caro-Kann. Also, it is best played with the Pirc move order (1.e4 d6 2.d4 Nf6 3.Nc3 e5) because it has been determined that White gets a big advantage in the Antoshin, Lasker, and Exchange Variations, all likes that come from the 1...e5 move order.
A book called "The Philidor Files" is probably the best resource.
The Tennison Gambit, on the other hand, is utter cr*p, and basically busted. It arises from either 1.Nf3 d5 2.e4? or answering the Scandinavian with 2.Nf3 via 1.e4 d5 2.Nf3? (yes, a full question mark - BAD - Not Dubious).
Black has a clear advantage after 1.e4 d5 2.Nf3 dxe4 3.Ng5 e5!! (Stronger than lines where Black tries to hold the pawn like 3...f5, 3...Bf5 or 3...Nf6) 4.Nxe4 f5! 5.Ng3 Bc5 with a clear advantage to Black!
Tennison Gambit is complete garbage! Look elsewhere!
Tonight I've been experimenting with some new openings. I've come across two that are similar. The Philidor defense and the Tennison gambit. How would you rate these openings? Are they viable option to use? What color do they work best with? Which one do you prefer if any? Thank you in advance.