Both are illuminating liquid highlighters that give your skin that extra glow. While Senna's Backlit is part of the HDExtreme range of foundations (comes in 5 colors and the pinky glow of Backlit), I'd be weary to wear it as an actual foundation. Fresh's Twilight is a more whitish glow and is actually a primer although, again, worn as a highlighter guarantees better results. Spot apply to brow and cheek bones.
Backlit (left), Twilight (right)
Senna's is a bit thicker in consistancy, while the Fresh Twilight has more of a liquidy feel, making it slightly easier to blend.
So, now, the only question remains: what color do you prefer? A pink hue or a white glow?
TIP: Mix a dollop in with your foundation for an all over goddess glow (like you've been backlit).