physics - When should I use a fixed or variable time step?

Should a game loop be based on fixed or variable time steps? Is one always superior, or does the right choice vary by game?

Physics updates are passed a "time elapsed since last update" argument and are hence framerate-dependent. This may mean doing calculations as position += distancePerSecond * timeElapsed.

Pros: smooth, easier to to code
Cons: non-deterministic, unpredictable at very small or large steps

deWiTTERS example:

while( game_is_running ) {
    prev_frame_tick = curr_frame_tick;
    curr_frame_tick = GetTickCount();
    update( curr_frame_tick - prev_frame_tick );
    render();
}

Updates might not even accept a "time elapsed", as they assume each update is for a fixed time period. Calculations may be done as position += distancePerUpdate. The example includes an interpolation during render.

Pros: predictable, deterministic (easier to network sync?), clearer calculation code
Cons: not synced to monitor v-sync (causes jittery graphics unless you interpolate), limited max frame rate (unless you interpolate), hard to work within frameworks that assume variable time steps (like Pyglet or Flixel)

deWiTTERS example:

while( game_is_running ) {
    while( GetTickCount() > next_game_tick ) {
        update();
        next_game_tick += SKIP_TICKS;

    }
    interpolation = float( GetTickCount() + SKIP_TICKS - next_game_tick )
                    / float( SKIP_TICKS );
    render( interpolation );
}