not necessarily mate.
The Aristo is the toyota name for the GS - as the Lexus brand does not yet exist in Japan (coming soon).
The main difference between Aristo and GS300 is the trim levels - the Lexus being the "luxury" version can have full leather etc.
There is a vertex version of the artisto which is twin turbo - but not all aristo's are turbod.
Unless you get this twin turbo version - there will not be any real difference in power.
One factor you probably want to consider is the cost of insurance. The GS is group 15 - so not necessarily cheap, and with Aristo's being imports, insurance can be more expensive.
They can also be harder to sell on, with some people being wary of imports.
Some nice models for sale on here - mudz has his up, and its in really good condition