I have and use the standard Hoya cir-polarising filter - very happy with it, no problems. The 'Pro 1' has coatings on it that are designed to cut down on flare and ghosting so should give a more contrasty picture. The figures and graphs that Hoya give are for the front of the filter so perhaps this is a problem if you want to use multiple filters at once. The most I have used is an ND grad(Cokin) and the polariser together and I have not had a problem with flare, ghosting or vignetting. The other difference they quote is the 'Pro 1' is a lower profile - again depends on how many filters you intend to use at any one time, also how wide the lenses are you intend to use it with. I don't think you would have any problem with normal wide angle, perhaps you would if you are going to the extreme wide angle. My feelings - the standard is suitable for all normal usage, get the Pro 1 if you are using the new generation super wide angle zoom lenses for C size sensors or any other extreme wide angle lens or multiple filter set ups.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html