I'll take a crack at this but I have no clue about what makes a t-shirt well designed. So pardon my ignorance!
CrisKre said:
Front:
I like how the cover is plain but has the ominous words "protect the light." It provides a unique bit of intrigue and the font makes it interesting. I would increase the font a bit (thinking a good two inches from the arm seam would be ideal) and move it down slightly (about an inch). I'd also create a SLIGHT more bit of noise in the font to pronounce the little extrusions you have. One last thing; I don't like how there's a period there seeing as it's an incomplete sentence.
Back:
The logo placement is perfect; of course there wasn't much else you could do with it. The Echoes part of the logo is probably a bit too dark and it'd be hard to read from a small distance. I'd suggest brightening that up or adding a bit of a white glow around it to differentiate it from the background more. The artwork is superb but I don't think it'd work well with the complimentary artwork of the other t-shirt. Although it's an excellent piece, the character Samus isn't in full form which is important in these character driven designs. I'd suggest finding a nice heroic post with her gun in the air to offset the more beastly pose of the other t-shirt not to mention you'd match the size effectively.
CrisKre said:
Front:
As for the front, my comments from the other t-shirt apply. Of course I'd keep the font the same size even though it takes up less space for sake of continuity.
Back:
The logo comments also apply to this t-shirt. The artwork you chose for this t-shirt is spot-on. It's the perfect pose and also eschews a bit of darkness with her stance and color-scheme. I'd get rid of the odd lines on the sides and eliminate any semblance of a border or random pixels. Another possibility is adding in the noise and extrusions you have from the font to the artwork. It'd provide a sort of an aura to the character; it would pronounce the otherworldly nature of this version of Samus. It might look good or it might complicate the piece too much so give it a shot.