Twash99 is the best offhand for Mandau 99, but i would only really go for it if you either also love dnc or thf is you absolute favorite job or you just have nothing else to toss your money into.
Mandau is already really good with STR99 dagger, and in some situations (when you dont cap ddex with STR99, but you do with Twash) even the lv90 Twash is equal and or better better then STR99.
It really depends if the DEX from Twash makes you cap ddex or not. If STR99 wont get you capped, but the twash 90 will, it will come out ahead of STR99, but mostly just slightly.
excluding mobs where hitrate is not capped.. because i dont feel like making another TP set/food/buff combination now for those (at work atm)... both same TP/WS sets (just useing current skadi TP set, Thaumas-like Exenterator (maybe not best set but just picked what came to my mind what is used), Toci/hecatomb/abbateur-like MS, no kehpri stuff)
Mithra with WHM present /DNC STR/DEX12/Dagger16,Exenterator5, Solo Exenterator, SA and TA Mercy, Haste, 5%Haste Samba, 10%Defdown, BoostSTR, RCB) just showing the %ish-overall-dps increase by the weapon which is stronger:
Twash 95 vs STR99
>Fodder: Twash lead by 4,5% (Capped ddex, fstr,cratio on both)
>DC Nightmare: Twash lead 6,4 % (Capped dex for Tash, both almsot cap cratio, capped fstr)
>Hydra DRG/MNK: STR99 lead by 0,2/1,2 (not capped ddex,fstr,cratio)
>Buffalo EM: STR lead 1,5% (capped ddex on both, both dont cap fstr, cratio)
>Bhukis: Twash lead 3,3% (capped ddex on twash, both dont cap cratio, capped fstr on STR99 but not on twash)
Twash 99 vs STR99
>Fodder: Twash 6,6% ahead
>DC Nightmare: Twash 8,51
>Hydra DRG/MNK/RNG: Twash 0,8/0,8/5,3
>Buffalo EM: STR and Twash 99 equal
>Bhukis: Twash 4,94 %
Twash 90 vs STR99
>Fodder: Twash 2,5
>DC Nightmare: twash 4,3
>HydraDRG/MNK/RNG: STR 2,8/3,4/0,7
>Buffalo EM: STR 3,6
>Bhukis: Twash 1,3
those values in spreadsheet change if you have different race or buffs or food or gear and you know..anything. (i didnt download a new one, its a few weeks old if anything changed) but it gives you an impression how "far" ahead twash is. Its best you download it yourself and intup the gear you have or the gear you want to optain as best sets and play around with it.
in general i would say Twash99 > Twash95 > STR99 = Twash90 (sometimes better sometimes weaker) If you already have Mandau 99 and Twash90 and you plan to raise twash to 95/99 i wouldnt bother with STRdagger. But if you already have STR99 and no twasher and have other jobs to gear aswell i would stay at that.