utahrc gave you some good pointers on assessing the true differences in impulse classes
here are some more thoughts on clustered BP v.s. single composite motor-
with a clustered design you have an added level of complexity in the ignition system, and maybe a bigger battery required
in a clustered design you run the risk of delayed or failed ignition of part of the motors----this would effectively reduce your peak thrust and may make stability (immediately above the launcher) a bit dicey
with a clustered design you tend to have more weight in the back end of the rocket, which changes (for the worse) the size or amount of fins required for proper stability
with a single mid- or high-powered motor you have the (relative) problem of powerplant availability-----I know this is no big deal for some guys, but in many areas there are no stores to walk into and pick up an Aerotech (or equivalent) and you are forced to plan ahead, mail-order, and wait--- with BP motors you can usually find them in a hobby store, in stock
with a composite motor you begin to get seduced into using the reloadable motor casing thing, which means your rocket is now carrying an additional $20-30-40 investment ---kind of painful when it lands in the top of a tree or blows away in the wind
launching a composite motor generally makes more noise, which can tend to attract the (unwanted) attention of local residents who call the police who tell you to stop launching--- it has happened to me!