I researched this for a while before settling on a digital scale. A lot of it has to do with how much money you have to spend vs. how much time you are willing to spend.
If you're flush, go buy yourself a Sartorius force restoration scale (>$1000) and happily trickle away with sub-kernel accuracy
Digital scales are generally (much) faster and less prone to user error, and have the potential to be more accurate than all but the most insanely expensive balance scales.
Both are subject to environmental factors such as changes in ambient temperature, vibrations and small air currents. Load-gauge digital scales will (generally) exhibit more drift due to temperature changes and require more frequent calibration, but are also subject to variations due to noisy power supplies and interference from nearby EM sources (cellphones are a common culprit). They are also much more sensitive to shock and overloading that can damage the load cell.
Some (most) digital scales also have a problem with trickling and drift. What's going on internally is that the load cell is sending a constant stream of updates to the management software about tiny variations weight. Aside from you actually putting something on or taking something off the platen, all of that is noise that needs to be filtered out. It's EM interference, or tiny vibrations you can't perceive, or slow changes in temperature, or dust mites humping in the next apartment over, or whatever. So the software is constantly trying to figure out what's the signal in all that noise that represents the 'true' weight on the load cell. This has two effects:
1) Your scale will tend to drift over time as the software corrects for all that spurious input and the adjustments begin to add up. So something you weigh at one point can weigh differently by a minor variation at another point. This isn't a big deal -- you just need to make sure your scale is warmed up before you use it (most manufacturers recommend a warmup period, but most serious reloaders seem to just leave their scales on all the time)
2) Trickling can be tricky because, if you trickle an amount that's smaller than the measurement accuracy of your scale, the software will say 'hey, a minor variation in weight. That must be due to a temperature change or something' and will recalibrate silently to eliminate what you just added. If you get a digital scale and trickle kernels of salt onto it one at a time, you can wind up with a heap of salt on the platen that reads WAY lighter than it should. Different scales exhibit this problem to varying degrees but, as a rule of thumb, you are generally OK if you trickle an amount more than half of one division in terms of its measurement accuracy. I use a GemPro 250, which in theory can reliably detect differences of a single kernel of powder, but I still zero it frequently.
Digital scales also can suffer from power supply problems-- you'll want to make sure any battery powered scale always has fresh batteries and make sure that, if your workspace has 'dirty' power, you run some kind of power line conditioner. That seems to be very specific, though, so don't worry about taking this step until you're troubleshooting existing problems.
Digital scales can be much more accurate because load cell technology is cheap, reliable and easily mass produced. So, for about $200 you can get a scale that measures to .02 gn precision-- that's generally less than a single kernel of extruded powder. The other reason they can be more accurate is that they don't rely on a human to read the scale. They have a nice LCD readout that tells you exactly what (it thinks) your sample weighs.
Digital scales can also be much, much faster. Again, it varies by scale. My GemPro 250 can take a few seconds to settle on a reading, but it's still generally going to be faster than a mechanical scale. Other scales are much faster, and it doesn't necessarily vary by cost. My $60 Frankford Arsenal scale is much faster to measure than my $200 GemPro (but the FA scale is only accurate to a tenth of a grain).
In spite of all the technical caveats, I chose to go digital because of the speed and efficiency. Others will have varying opinions, and some will get religious on you
