I use three different types of Port glasses but as far as I am concerned they are all able to go with the various categories of Port.
a. The "official Port wine glass of the IVDP" was designed by architect Alvaro Siza Vieira at the request of the Port Wine Institute (
http://www.ohmdesign.com). One of its unique points of difference is that it has a quadrilateral stem that makes the glass much easier to hold onto compared to a round stem. Additionally, there is a notch or "dimple" in the stem, which is tailor-made for a thumb or forefinger. It fits very comfortably, adding stability, which is especially important at a tasting where you are likely to be jostled. There are other cool features of this glass (like the flat profile of the base) that separate it from its competitors.
b. Riedel Vinum - this is Port glass I use most often. It does not fit as comfortably in the hand as the aformentioned glass style, but I find it provides the perfect balance to the aromatics and flavors without highlighting the alcohol.
c. Spieglau - The taller and narrower profile of the bowl, makes this less perfect in my opinion. The alcohol is defintely more prominent in this glass than the other two, and if you have a wine that already has this as an issue (say Graham's 1977 VP) then I would suggest another glass. Nonetheless, for casual every day drinking, especially for a Tawny Port, this glass does suffice.
My two cents on the topic.