BMW's are certainly better built to be honest, find them more reliable, but if you want to fix something it costs you big style unlike the omega to be honest.
Both have many pro's but not that many con's 
They aren't, they have strange ways of improving
perceived reliability - for example, the engine management light doesn't come on when the ECU detects errors

I come from a family of BMW owners, I know how 'normal' they are when it comes to reliability. Additionally, to make matters worse, their dealer network makes Vauxhall's look stunningly good, even makes Renault's seem OK!
The BMW dealer in Bury St Edmonds, which my mum used to use, changed her HG 3 times (twice at her expense) due to an oil leak, even though before it went in the first time, she told them it was PAS fluid (my brother had already checked).
The BMW dealer in Oxford did the first service on my brother's £70k X5, then decided to put it through a carwash. As if that wasn't bad enough, they left the sunroof open! They didn't tell him, as if they expected him not to notice the inside was full of water. They wouldn't even give any discount on the service.
An the BMW Mini, well, their reliability doesn't even need to be questioned. I know many Mini owners who have been given a brand new car as they cannot fix the issues with the car. To get the new car though, you have to sign a disclaimer that there is no warranty whatsoever with the new one.