It is the same level of details. All 7.15 maps are based on TeleAtlas 2007.10 map release and all 7.10 maps are based on 2007.7 release. The sets TomTom sells are just smallers parts of the same mapdata. As someone here said, it's just marketing.
The TomTom description of map details just suck. The map provider TeleAtlas uses actually much more numbers.
First they state total streen network coverage, which for example for Italy is 100%.
Next they state fully attributed roads which practically coresponds to house number range, which for italy is 95,80 %
Next they state basic attributed roads which usually means roads without streen number information (you can select street but not home number in your TomTom) which for italy is 4,2% for 2007.10 release.
TeleAtlas also tells you in their datashets wether maps have RDS/TMC support, amount of postal code coverage, signpost, lane information and tourist road options.
Anyway I've checked the countries you are interested in in the TeleAtlas 2007.10 datasheet and all the countries you are interested in (Austria, Germany, Italy, Switzerland) have 100% road network coverage with 95%
or more address coverage.
Wether you choose to uy whole Europe or just the smaler package is up to you. You will receive the same data.