You know ... if you are going to go submersible, why not go with a titanium heater. And while i have heard great things about vistherm, i have 3 other companies that i think stand above vistherm (but this is just my opinion).
In number 3 would be Hagen heaters. I like these because all of their heaters have an extra item that surrounds the heater so that a ) the fish can't lay directly on the heater and harm them and b ) if the glass shatters, it will mostly be contained. I have used this heater in a past tank until i sold it off (worst mistake EVER!!)
In the number 2 spot i would have to say Ebo Jager. These are REALLY relieable heaters and again, totally submersable. I personally use an ebo jager in one of my tanks and it always keeps the water +/- 1 degree fahrenheit (unless outside conditions have an impact.)
In the number 1 possition i have to say a Won Brothers Titanium heater. You can get these off ebay for a really good and reasonable price! Now this is hands down my FAVORITE heater because it is a titanium one (it will never break/shatter), it is totally submersible and enclosed, it comes in various model types for what you are willing to spend (i personally have the Won Brothers Pro Heat II), the model i have comes with your thermostat by your plug, this thermostat not only allows you to set the temperature you want without getting your hands wet, but it also has a digital readout of your current tanks water temperature. This temperature is read by a seperate probe that is also placed in your tank 12 inches away from your heater (just so that it doesn't give false readings). It is easy to use, and is extremely accurate (+/- 1 degree fahrenheit). When i have to get a new heater, i will ONLY go with a Won Brothers heater. They are well worth the money you will spend on one!
Lunchbox ... "i'm a man, but i can change, if i have to, i guess"
Tankless ... for now ... until i find what i really want (and what the wife lets me get!)