Ebo-Jager is probably the best submersible heater. 50-100W should be plenty for a tank that size unless it gets really cold in your room.
IIRC, Ebo-Jager is no more. I don't remember all the details, but I think Eheim took over building the Jager heater line a few years back. (Ebo built the Jager heaters for decades)
Since they're now all made in China, quality has gone in the crapper for all brands. Ruby is right...Ebo-Jager heaters were the best you could buy for many years. I still have a couple of 100w and 200w Ebo-Jager heaters that were built in the late 80's and early 90's that work as well today as they did when they were made...
I used Marineland's Visi-Therm line for many years, but they've also gone downhill badly.
IMO, stay away from the titanium heaters. The build qualiity on those was horrible when I got out of the hobby a few years back.
If I were buying a heater for a 30 gallon tank, I think I'd still look at the Marineland Visi-Therm line...and maybe the Marineland Stealth line.
Hydor MIGHT have gotten better since I got out of the hobby. They used to be pretty unreliable.