The lifespan of a steam generator is dependent on many factors such as water quality, frequency of use, length of use, and frequency of maintenance draining, to name a few. Some generators may last only 3-5 years under extreme conditions. Then again, some have lasted more than 30 years.
You can certainly contribute to a longer lifespan with preventative maintenance through routine draining of the unit and filtering the incoming water if there are water quality concerns. The automatic drain system, available on most units, is a must-have.
As far as which generator series to choose, select one of the industry's leading manufacturers (Amerec, Mr Steam, Steamist, ThermaSol). Then the decision boils down to features and appearance. Which of these features are most important to you? Control and steam head/outlet finish, control features, coordinating light and sound systems, warranty, generator size and shape, drain system method, and tandem requirements are the leading determining factors for most people.
Start by sizing your generator, try this tool... www.steamsaunabath.com//catalog/generator_sizing.php
Then use the handy comparison tool (just select your top choices and click compare). When you have a good idea of what you like or some questions, give these guys a call and they'll walk you through it.