ok crap that didn't work
If I could transform into something, I would choose to be able to transform into and back out of a Spartan warrior whenever and however many times I feel like. Spartan warriors have superior physical strength and fighting abilities when compared to modern day warriors, because they spent their entire life training and learning how to fight effectively. Combined with modern technology, these warriors would be unstoppable. Their determination to win their battles are unmatched. I would transform into my Spartan self when the need to prove myself superior requires the need to prove myself physically, as opposed to mentally as my current self can accomplish most of the time, at least in fields of math and science. Nobody needs to prove themselves in English though, so that does not matter to me.
Spartans are the epitome of manliness. This would be an advantage to me, because my current body, while very manly, is not even at the same level of manliness that being a Spartan would help me accomplish. It is a primal urge of mankind to prove their superiority over other men. The ones who can accomplish this through physical superiority, and therefore the ability to acquire members of the female gender, are dubbed the manliest. I am a man, and therefore cannot escape these primal urges. However, proving oneself through mental superiority only is not an effective way to prove yourself over other men, because those with a smaller mental capacity (the ones who I would be proving my superiority over) only care about physical superiority, because they do not have the ability to comprehend the advantages which mental superiority brings.
There are some situation in which being a Spartan is not ideal. Such cases include working on a computer, or taking a test, or scratching ones back. The bulging muscles of a Spartan's arms make reaching those hard to reach areas even harder to reach. In this case, I would only have to say "This is madness" to transform back into my ordinary human state. If I ever need to transform back into a Spartan warrior, which should happen very often, unless I stay in Spartan form for an extended period of time, I would simply have to shout at the top of my lungs, "THIS IS SPARTA!!!!!!" This assures that I can be in whatever form I wish for as long as I wish, being able to avoid the negative consequences of each shape by transforming into the other shape.
If I could find just three hundred other people who are able to transform into Spartan warriors at will, we would be able to almost ward off any Persian invasions of the United States. Granted, there have been no Persian invasions of the United States in recorded history, however that does not rule out the possibility of one in the future. It is a known fact that a Persian's one weakness is a Spartan warrior. As a Spartan warrior, I would better be able to protect my homeland from those possible future invasions.
Being able to transform into a Spartan warrior at will would be my metamorphosis of choice. The physical superiority, manliness, and Persian-repelling natural abilities would be of great advantage to me, and the disadvantages would easily be countered by being able to transform back into human shape. Is this madness, you say? Well I respond, "Madness? THIS IS SPARTA!!!!"