As far as HP, it means very very little when it comes to vacuums. ALWAYS look for the AMP rating on the unit, you will probably have to break into the packaging to find it near the UL listing stamp on the unit (often near the power cord).
No 110v appliance can pull more than about 1650 watts continuously from a 15 amp source; that's equal to 2.2hp. Under load, inductive motors often pull >2x their continuous rating. Obviously, no 6 hp vac is ever going to pull 40 amps from your wall (which is what would be required to ever achieve 6 hp). I really don't understand the ridiculous HP marketing strategy; the I/C engine companies (Briggs) got sued for similar misrepresentation and lost very recently.
Motor aside, the fan design in the unit is going to largely dictate how well the vacuum performs. Purchase a recommended brand and you can generally count on decent engineering and performance.