Options
200 watt load (highly recommended)
Even if you don't currently play higher wattage amps, what happens to many guys is they realize they can play big amps for bigger tones with the UA without worrying about having too much power or amp. It's better to be safe and err on the side of more power handling rather than not enough. It also adds to the value of the UA.
100v tap (highly recommended)
The more you crank your amp, the hotter it gets. That includes all components including tubes and transformers. The 100v tap protects your investment by running components cooler which can help tubes and components last many times longer when cranking the amp.
Vintage amp players will also appreciate the fact that their amps were made to run on slightly lower voltages, as voltages in the US have crept up about 10% since the '60's and '70's. No rebiasing is needed and the voltage drop is not dangerous to tubes or components. Actually, the opposite is true.
*If you're running a 220 - 240v version then the tap is approximately 200v to keep your gear safe.
Plexi switch (recommended)
The plexi circuit is a reactive component of the load and helps normalize (match) the presence response in cranked amps. When switched out, a slightly darker tone is the result. The plexi switch is great on it's own but is highly recommended with the bedroom switch as you can hear the inductor "sing" the note with cranked amps (especially higher wattage amps) at lower than conversation, bedroom levels.
Bedroom switch
A must have for home players who live with others or late night jammers. Can get ridiculously low volume levels.
Variable line out
Useful for direct recording or wet\dry rigs, enabling the original tone to stay pristine while adding fx.