Personally, I'd have to say Nitemare's is the best, although both of them have parts that they could improve in.
Nitemare's - like Kittie said, the motion blue captures the essence of Sonic's speed. But my only problem is that the background lacks depth in my opinion. Personally, I would have liked to see more colors of Sonic in the background, such as red or green ( color of Sonic's eye ). So looking at it, it seems to me that it looks a little boring. The text could use a little more work too, but it's good.
AppleDew - Personally, text like that for a watermark is way too big. I would have to say like a small serif font or a pixel/bitmap font would be perfect for a watermark. Since this is like a Sonic-theme GFX battle, I would have to say you need to focus more on the render. The background doesn't support the render, not giving it any life. Plus, I would've liked to see the render a little more brighter too so it could stand out. The effects are alright, and I also would have liked to see a little more color too. Also if you look at the render, it's facing like at a side-ways direction, so for these type of renders, you wouldn't place them in the center but a little more to the left or to the right ( depending on which way the render's facing. )
So great work to the both of you.