it's a multi-variable equation, so the question depends on:
-what ws you're using
-what your ratio is before ammo (and mob def)
-how much WSD you have already
for just wsd vs fotia, you can calculate by looking at WSC
savage blade at 2000 tp has 10.25 ftp on first hit and 1 ftp on second, 11.25 total.. adding 0.1 ftp gives you 11.35 .. 11.35/11.25=a 0.88% gain
fast blade II at 2000 tp has 3.5 ftp and ftp replicates, so 7.0 total.. adding 0.1 ftp to each hit gets you to 7.2, or 2.86% gain
6% WSD from knob if you already have 65% WSD is 1.71/1.65 = 3.63% damage on first hit.. for savage that'll be 10.25/11.25 of the sum damage or 3.31%
for fbII first hit is only half the damage, so 1.815% from knob
in this example, an imaginary fotia ammo beats knob for onion sword II but loses for savage blade (but it also isn't factoring the attack on knob)