Hi orestis,
You have to carefully read what is being refered to in this thread. It is related to the technicalities of the focal ratio, not the actual size of the scopes.
If I had both the 8"f/4 and the 13.1" f/4.5 at the same magnification, the 13.1" 's image will be brighter, hands down. It is just a relationship between the focal ratio and the focal length of the eyepiece.
If the same eyepiece is used in two scopes of the same size, but one has a longer focal length than the other, the image in the shorter focal length scope will 'appear' brighter, but in turn the object will be smaller in size (less magnification). If both scopes are at the same magnification, the image in both scopes will be just as bright.
In your case, the rule in astro is 'get as big a scope as you can afford'. That means, the 10" dob.
Wavy & Starkler just helped me understand this little quirk of optical properties.
Ta gents,
.
Orestis, if you can, get to a star party of a local club or one of those listed in the 'star parties' forum, and see what different scopes offer performance wise. If Katoomba is not too far for you (or your mum or dad), and weather permitting, I'll be at the Katoomba Airfield this coming Saturday night. The same three scopes mentioned below will be there, plus what ever others that come are bringing. Bring your scope and binos too, and a parent. We'll get your 130 humming too.