When choosing a computer to buy the most crucial thing should be value for dollar.
While it seems important to some to have the best of the best; whether it be the processor (type or speed) and others a logo (dell, apple, ibm, intel, etc.) the most logical thing is the value for dollar.
To me it is not prudent to empty my bank account for any apple product when they can be emulated using a pc configuration (google hackintosh for more details) or pay extra for a graphics card I don't need for business use.
A guarantee is pointless in my view as the majority systems will be out benchmarked before the hardware starts failing on you.
Tomygun is right about one thing; deciding the use for the system. Do a little researching on what will fill that need at a the right cost!
To summarize I feel it is crucial to do a little research and get the best value for dollar on the system that fits your needs best.