Hi Orestis,
As Alexander said, if cost and portability are not huge issues, the 10" wins over the 8" every day of the week. A good big 'scope will
always beat a good little 'scope in light-gathering power and with that the ability to "see" increasingly small and increasingly faint objects. The larger 'scope (which will almost always have a longer focal length) only misses out to an extent on extremely large deep sky objects like the Pleiades etc because it just hasn't got the native field-width to take it in all in one go.
Also, make sure you have a read here:
http://www.iceinspace.com.au/forum/s...ighlight=ratio
I normally try very hard not to do hard-sell advertising here for the magazine I write but, I will make an exception on this occasion.
The new edition (April 2010) of Australian Sky & Telescope (in the paper-shops next week) has a superb, detailed article written by Tony Flanders on choosing a telescope/binoculars for beginners. It is near the back of the magazine.
Honestly, it is the best I've seen for a long, long time if not ever on the topic --
mandatory reading for first-time telescope buyers.
Best,
Les D