The earlier launch for sure was a big advantage but it was also more needed for success than this gen.
360 came after XBox and ps2. The ps2 got an endless stream of 3rd party exclusives. People couldn't buy a 360 and expect to get as much games as they would get on ps3. But the 360 became a huge success and everything became multi. This gen everyone expected to get the same games on both consoles.
The price advantage is also often made a bigger point than it often was cause it shrunk fast and varied through time.
~Half a year after the european launch Sony introduced the 40GB model for $400€, the 20GB 360 did cost $349€ and the 120GB $449€ at the same time.
And you could change the hdd for the ps3 at will cause it was a "standard" hdd, instead of paying the ridiculous price Ms asked for.
PSN also was for free, making the ps3 actually cheaper if you played online.
The EU price did not matter as much because Sony was winning there but the UK .
It was the 360 coming out 16 months before PS3 in EU that allow it to get that much sales to begin with in EU .
Which is why people were saying if they came around the same time and near the same price MS does not stand a chance in EU.
Also the price advantage was not a EU thing it was a WW thing .
For eg it took Sony till 2009 to have a $299 PS3 in the USA .
MS had one since they launch in 2005 plus a year head start .
I really don't see how you can say the price advantage was not a big deal when they had a big one over sony for 3 plus years .
Mind you this also during the time that COD blew up making the 360 the go to console in USA.