Would you rather have the power to..

There is no in-between, you can either be at normal or maximum, or normal or minimum density.

Your ability affects your entire body and cannot be used only on chosen parts. This does not affect your internal physiological functions.

With increased density, you become durable enough to tank bullets and stuff up to small explosions, like grenades. You become extremely heavy, of course. Heavy enough that if you were dropped on a car, you'd bust through its roof and heavily damage the chassis underneath. Your strength is not augmented, however, so while your density is increased, you slow down to about 1/20th of your normal speed when really exerting yourself to move.

With decreased density, you become lighter than air. This allows you to float and basically "swim" up off the ground and around. You are, however, subject to the whim of the wind. You can "swim" up to against gentle breezes, but you do not have the power to fight against a moderate gust. Although your density is altered, you keep your shape, so you can phase through chain link fences, screen doors, barbed wire, etc., but you cannot squeeze yourself through a small crack in a wall. Small objects, of course, will just go through you, but solid objects that are about half your size or bigger will push you out of its way.

The effect is instant when you activate your ability, so to truly maximize this power, you can get creative with proper timing. For example, you can time your density decrease to ride the wind for faster travel, or time your density increase to take advantage of momentum and add more weight to your punches, kicks, or tackles.