BitConverter.ToString() vs Convert.ToBase64String()

I had thought that Convert.ToBase64String() was the method to use to create a base64 string of a byte array, but I recently came across BitConverter.ToString(). What is the difference between the two?

And more specifically when should one be used over the other?

For example in this question about creating a MD5 digest, a comment by CraigS on an answer states "ToBase64String doesn't return what I want. However, BitConverter.ToString around the byte array does the trick."

BitConverter.ToString(
    MD5.Create().ComputeHash(Encoding.Default.GetBytes(StringToEncode))
).Replace("-", "")

vs

Convert.ToBase64String(
    MD5.Create().ComputeHash(Encoding.Default.GetBytes(StringToEncode))
)

Also, what should be used to encode images to base64?

public string ImageToBase64(int Img_ID)
{
    byte[] tempBytes = showImageById(Img_ID); // get image from DB
    return Convert.ToBase64String(tempBytes);
}

vs

public string ImageToBase64(int Img_ID)
{
    byte[] tempBytes = showImageById(Img_ID); // get image from DB
    return BitConverter.ToString(tempBytes).Replace("-", "");
}