No experience with the FLIR tk but at 600 bux for a thermal I can't imagine you're going to get clear identification on anything that isn't close up. If you're looking for blobs in the distance then switching to your Photon then I guess it wouldn't matter too much though.
I myself like the pulsar line. You get a great value and pretty good features as well but the cheapest models still are about $2k. I just got a hd38a that I'm looking to return for the 19a so I could have more FOV. The natural 2x is nice since its easier to identify targets clearer at distance but the sacrifice in FOV isn't worth it for me. I'd rather have a quicker time acquiring a target, then switching to my scope for a clearer look.
Basically, thermals compared to no thermals of course would be quite the advantage, better than just using your Photon to scan. But I would try to save up a little more and go with the Pulsar Quantum line. I know someone selling a used hd19a for like a grand, PM me if you want the details.