Which one will have a better soundstage? Unfortunately we can't test it out together as no one has both speakers in store...
Different audiophiles have different definition of what sound stage is...
Am I right to say that S2000MKIII has the best performance to price ratio for Edifier line of products? Performance wise better than S2000, S2000 Pro, S2000 MKII, and price to performance better than S3000 and S3000 Pro as it is cheaper?
Hard to compare with past products since newer models are intended to replace older models at the same price, while older models get cleared. OTOH if you bought the older model at the launch price you have already enjoyed the product during the time frame so consider it as paying for the enjoyment.
If the MKII (or its other variants like Pro) and MKIII are at the same price I would get MKIII.
S3000 is pretty much the same thing as S2000MKIII except one size bigger, so the usual pros/cons of bigger size applies here. On the desktop, S2000MKIII sounded better while being easier to position. With longer listening distances, S3000 is better.