The Tetra one is a HUGE joke. Seriously. It had such terrible reviews, but I really needed my 10 gallon heated, so I bought the 100watt and thought I'd live with it. HAHAHA, ya, right! 100watts for a 10 gallon and it has successfully brought it from 68* to 72-73*.. the joke? It stops heating there, maintains that temperature, and even shows a green light which means it's at the right temp, HA! I put the heater on in my apartment and it's now at 75*, so I know this heater just sucks.
That little betta heater from MissM does a terrible job, too.. don't ever try it! Mine has never once gotten warm in a proven light socket after days... The heater itself NEVER warm! But the PetCo Submersible one I've never tried.
I ordered the Eheim Jager 25watt for my 10 gallon, and if I like it, I'm buying it for my smaller tanks. I'm done with heaters!!