SD decrease rate: the damage absorbed by the opponent's shield in PVP directly translates into HP damage according to set value. So if SD decrease rate is increased by 5%, the opponent's shield's absorb rate is decreased from 90% to 85%, and the HP damage the opponent receives directly increases from 10% to 15%. In short: SD decrease rate affects each your hit, but with a small damage improvement.

SD ignore rate: the rate of ignoring opponent's SD and giving HP damage increases according to set value. So if SD ignore rate is increased by 5%, this means that you've got 5% chance (for one hit) to send all your damage directly into opponent's HP. In short: SD ignore rate depends on your luck, but with a huge damage improvement.

Base damage means damage made by a weapon without any yellow option. Each of the damages are divided into two parts - HP and SD. Our point is to deal as much as possible damage to HP. The first table is for Battle Master, for example. His weapons can have maximum 10% value of each of these yellow options. So, SD decrease rate 10% means that the opponent's received damage will be divided as 20%/80% for HP/SD instead of 10%/90% standart. And SD ingnore rate 10% means that each 10th hit will divide the damage as 100%/0 for HP/SD. The second table is for Grand Master, because his staffs can have even 15% improvement.

So, these options are equal, but Blade Master can have each in his pair weapons, and Grand Master shall make a choice. SD decrease rate works always and for each strike and if you prefer reliability, you should choose this one. SD ignore rate works only for 'successful' strikes, so this is good for lucky players. Also using SD Ignore on classes that have high SD by auto would be a smarter choice to rely on luck and deal high damage at once.