With the GSO the diameter of the tube is just to allow more of the 75% light path. the Skywatcher is better for astro photography the Pyrex with its low CoE means you don't have to worry about the mirror expanding like the glass of the GSO with BK-7 glass, but like i say its not made for astrophotograhy so be prepared to modify it to work properly!
Not to put you off it, but to do it properly your looking at at least 3-5k straight up to get a basic kit together from scratch. Skimping along the way will lead you to spend more in the end.
have a look at my website
www.brendanmitchell.net for a look at some of the photos i have taken and the equipment i use.
there are also more photos at my photobucket site.