Almost all of these devices are based upon a technique called force restoration where the unknown weight is actually picked up or supported by an electro-magnetic force motor.  The force motor usually sits in the center of a suspension system that was first invented by the French scientist Gilles de Roberval in 1669.  This suspension eliminates the effects of side and end loading (where the unknown weight is placed near the edge of the weigh pan) but requires the use of thin metal flexures as pivots.  Be advised, these somewhat fragile pivots are the most frequent source of failure in these units, usually due to inadvertent overloads.   This could be seen as reason number one for buying the highest capacity scale that gives you the resolution and accuracy that you need to do your job (two characteristics you need in your spec.).

 The force motor, which uses a very strong permanent magnet and a high permeability magnetic material, is very sensitive to small changes in temperature and quite prone to drift over time as stresses in the structure are relieved.  Frequent recalibration is required in high accuracy projects.  Consider a feature, commonly called automatic internal calibration, which automatically recalibrates the scale (when not in use) if the internal temperature of the scale changes by more than a customer defined amount (usually +/- 1 degree Celsius), or if a preset amount of time passes without a calibration cycle.  Many people have looked back with regret because they did not make the initial investment in this feature.  

Another area where hindsight can be painful is the area of scale capacity.  We have already made the point that overload is the most frequent cause of mechanical failure, and having larger capacity helps.  Many scale manufacturers produce several scale capacities based upon exactly the same electro-mechanical package.  Only the amount of labor spent in adjusting (tweaking) the scale varies, and so does the software (no cost difference).  Testing time may also increase.  However, a 400% or 500% increase in capacity may only produce a 10% to 15% in crease in price which is especially significant if the absolute value resolution and accuracy (linearity) do not change over the range.

Consider investing in a graphics (pixel type) display rather than a segmented type.  The cost differential is usually small and the rewards may be very large, things like on-screen instructions, or graphed results.  Look at the number of applications offered, and the number of weigh units in which the scale can display results, even though you think you don’t need them now tomorrow may surprise.

The keyboard is your primary input device and has a lot to do with your satisfaction in using the scale.  More keys generally means less multiplexing of keys and easier operations.  Two tare keys are more convenient than one.  Navigation keys make things more intuitive.  Numeric keys make data entry a lot simpler.  The ability to use a pc type keyboard for alphanumeric entries is a blessing.  This is one area where more means more.

Do your homework; it can save a lot of regrets.