If you do a search you'll find many differing opinions, basic rule is you get what you pay for...
A good vac will suck up anything that will fit up a 50mm nozzle end, larger pieces of brick and slate can be held on the nozzle by suction so you can drop the pole down and remove it.
We use omnivac 4200 - I understand sky vac's are also very good. Make the extra investment in getting carbon poles
Darran