I had a Mk 6 GTI for a couple of years (09 reg). I had heard that the Mk5 had some questions over 95 fuel, but it never gave my one a problem, probably best to check it out on the VW forums though.
The Mk6 was supposedly more efficient than the Mk5, but with hot hatches and current petrol prices, thats all relative. A full tank would rarely register over '300 miles remaining' on the trip computer, so not one for long commutes, unless cash really isnt a problem at all.
On the plus side, lovely car (i think the mk 5 and 6 are very very similar performance and interior wise) and the five door i had did a great job of being a toy and a 'family car' in one. There was a special edition MK 5 at the end of the run, (Edition 30?) that was a bit pepped up if you want more oomph without going to the R series.