In my experience, .mp4 looks too blurry for high-quality gamedev gifs, especially since a lot of them incorporate a little bit of retro style - .webm does a much better job on this. My preference would be Gfycat, because it provides both .webm and .mp4, so the tag can default to the higher-quality .webm version and fallback to .mp4 for mobile and Internet Explorer. Imgur on the other hand only supports .mp4.
This blurriness you're talking about really depends on the mp4 compression. Surely, webm can achieve smaller size for better quality (better compression rate, which much less quality loss). The thing is: you can feed gfycat with a non-compressed mp4, the webm generated will still look nice. Although I cannot speak for imgur's gifv, but I think if the mp4 is generated from a gif, the 256 color restriction makes the compression do less harm to the final quality of the mp4, so less blurry.