No,they have their own versions of apps (e.g.Angry birds for the htc sensation can't work on the xoom).
And the reason is very simple,because of the different android versions used on those devices,as a device running android 3.0 can't run apps intended for a device running android 2.3 (that's because android 3.0 only runs on tablets not phones) and vice versa.
A device running android 2.2 can't run apps intended for android 2.3 and vice versa.
But you shouldn't worry about that as all the good apps exist for almost every single android version out there (Angry birds for example).
And to the question about which is better,a tablet or a laptop?
Well,a tablet is better when it comes to on the go usage,and battery life,and they can do everything u need to do quickly when there's no computer,mac or laptop around.
A laptop on the other hand is not good for on the go usage or browsing the internet,and u need to be near a power source almost all the time (u can run about 8 hours using the mac airbook without having to plug it in,but u can't say the same thing about Dell).So a tablet would be the perfect option for "out doors" or "away from home" usage
So,if u ask me which i'd buy? it would certainly be a tablet not a laptop.But that's just me.