build and component quality will affect longevity. go by CEA ratings for power and you'll know what they really put out so you won't need to fear over or under-rating of output.
cheap brands tend to over-rate power by using peak power ratings. better companies often tend to state specs that are a bit below what their amps really put out, so it may say its a 1200 watt amp but the birth sheet will actually read something like 1430 watts.
features and build quality are the two main things you buy with a good name though
I agree with GlassWolf( The most suggested "Best Ecoustics Member") He is very knowledgeable and can help everyone with anything!
I fell he left one category out though and I would like to add a little as well:
Cheap brands (like Volfenhag) rate their amps at higher voltages(like 16-18 volts) or use their peak power output ratings to increase the stated output.
Budget (like hifonics) rate their amps at a the standard voltage (14.5) and slightly overrate their amps
Better (like Kicker/Rockford) slightly overrate their amps at standard voltage (14.5)
Best (like Crossfire VR series) overrate their amps by testing them at 12.5 volts and letting the extra 2 volts (to get to standard 14.5) create 352 more watts rms at 1 ohm. This is why their VR1000D is rated at 1000 watts RMS at 1 ohm and at 12.5 volts. The birth sheet says it is 1352 at 1 ohm at 14.5 volts.