I'm in the middle of building an RV8 (just couldn't resist the option to sit on the centre line!) so my tuppence worth on which would be the most enjoyable building project.....
The RV is an all metal aircraft so the bulk of the building effort is taken up by assembly, drilling, deburring and rivetting. It's fun and a generally dry, muck-free experience. From time to time (sealing fuel tanks, priming parts, applying an interfay compound) you have to get out the rubber gloves, breathing equipment and goggles and mess with sticky wet stuff which is potentially hazardous to your health. My only knowledge of building a Europa comes from the 'Plane is Born' TV program so I don't have any first hand experience, however the impression I have is that there is a much higher 'wet' content to the build than with a metal aircraft. So for the choice of which to build, for my money if you want to build a Europa you have to be happy to spend a lot more time working with the mucky stuff. Personal taste!
On the other questions.. I have no idea about relative running costs, maintenance, insurance etc and have not flown a Europa to be able to comment on how they fly. The RV however is a true delight to fly with very crisp control response, amazing climb performance with even the smallest of the engine options and excellent cruise speeds.
While I am on, the RV7 and 9 kits are stunningly well put together. So is the -8 but if you choose a slow-build -8 fuselage it is a lot of work. The key to the ease of the build and the eventual quality of the finished product is the fact that the kits (apart from the -8 slow-build fuse) are mostly pre-punched with matched holes so you can effectively get the parts out of the box and 'snap them together'. There is of course more work involved between doing so and finally rivetting the thing together but it is very satisfying how quickly you can turn a bunch of parts into an aeroplane part!
GRP