not sure about Obi-Wan igniting his saber before Vader ignites his, it just doesn't really seem to be very Jedi-like, it makes him look like he's on the attack rather than the defense, it also makes Vader's 'I've been waiting for you ' line seem out of place, as it almost looks like Obi-wan was waiting for Vader to arrive.
It doesn't make him look like on the offence, just prepared, because he know what Anakin has become. Vader walks to Obi-Wan and attacks! I don't think Obi-Wan was waiting for Vader, but I'm almost sure he knew that it might happen.