The questions you are asking are very good ones - as are the answers you are getting. You might want to look at the last paragraph of the first post in the thread:
http://forum.lowcarber.org/showthre...&threadid=33561
As far as the best scale - get a medical balance. They can be had for about $60 to $100 and are worth every penny (IMNSHO).
One thing you DON'T want to due is make the assumption that digital scales are better than analog scales. Some are and some aren't. It's actually easier to make an good analog scale than to make a good digital scale at the low end of the price spectrum because cheap digital meters use a single strain gage instead of the four you would normally use to obtain proper tempurature compensation and to minimize bridge non-linearities in the sensor circuit.
The real question you are asking - and which several people are answering - without knowing it is whether accuracy or precision is more important in a scale for your purposes. Accuracy and precision are NOT the same thing. Accuracy means that, on average, it tends to give the correct answer. Precision means that it tends to closely repeat whatever answer it gives, irrespective of how close that answer is to the correct answer.
Since you are looking for changes in your weight - you want an instrument that has high precision. Don't worry if the answer is not very accurate, as long as you get the SAME answer whenever you are the SAME weight.
So how do you find a precise scale? Test it for variability. Step on it and off it several times. Put your feet in different positions on the platform. Vary that rate at which you transfer weight onto the scale. Look for the scale that gives you the smallest range of values as you do this.
Now, once you have your scale, you can further improve the precision by stacking the deck in the scale's favor. Always transition your weight onto the scale smoothly and in the same way each time. Always put your feet on the same place - put some tape on the platform if necessary. Now repeat the variability measurements while following these procedures and you will get an idea of the tolerance you can expect on any given weighing.
But other factors affect the precision and accuracy as well. Keep the temperature in the room where the scale is constant (say, within 5 deg F if possible). And you should always weigh yourself with the same clothes on (preferably with no clothes on) while you are completely dry and preferably first thing in the morning (after going to the bathroom if needed).
This will minimize the variables and make it more likely that weight CHANGES shown by the scale (and always use the same one) probably represent accurate weight CHANGES in your body.
But that still leaves the issue of properly interpreting the meaning of those weight changes - especially short term ones. That's a whole different topic.