Is a Roadie with wider tyres still faster then a MTB with slicks? I need a new bike, but I´m in a dilema on which too choose, because my rides involve 4 kinds of pavement: smooth cycle ways, cobblestone, ok city roads and rough city roads.

Was thinking just to keep doing what I have been doing till now, get a new MTB, a BTWIN rockrider 500(for those in the US, BTWIN is a brand sold in Europe that as nice groupset parts for the price) and fit it 26 slicks.

But then on the site saw they have some flat bar roadies: http://www.btwin.com/en/road-bikes/fitness-road-bikes/18701-triban-520-fb-road-bike-black-red-white.html

So I wonder...
If that roadie was fitted with 700x32 slick tyres would it make it rideable on cooblestone/cracked pavement without the risk of damaging the wheels?

And maybe the most important, would it loose much speed compared with the original tyres(700x25) on the smooth roads? Would it be just about as fast as the "slicked" mtb?

And please, no need to tell me to get a city bike. From what I have seen, they are even heavier then MTBs, and in my prices budget I can get a Btwin MTB with Sramx4 or Acera. While urban ones only have the tourney.
I'd recommend a Cyclocross bike (also called CX).  That's a robust road bike frame, configured to accept tyres up to about 40mm wide.  You won't need 40mm unless you're doing proper off-roading, but a nice 30-35mm file tread such as a Specialized Trigger or Schwalbe Sammy Slick will run nicely on smooth pavement, will handle cobbles and bad pavement, and will also handle mild gravel/dirt.  

I have a setup like that, and the thing that I love about it is that I can ride pretty much anywhere from road to gravel to bike paths to rail trails to single track (except for MTB trails with lots of rocks!).

Have fun.