I am not 100% sure what rotation you're using specifically, Zhirra, however Im assuming it opens with an initial application of IT, then utilizes Rime procs + the HB glyph to maintain FF on the target? Please correct me if I am wrong on this.
Assuming singe target, because AoE HB will win out hands down.
Utilizing
Hellscream Slicerx2. Using the aforementioned rotation. You would be hitting for (not accounting for AP and with rough napkin math)
One Disease
MH: 511+467.2*1.125 =~1100.48
OH: (511+467.2*1.125)*.5 =~550.24
Total:1650.72
Two Diseases
MH: 511+467.2*1.25 =~1222.75
OH: (511+467.2*1.25)*.5 =~611.375
Total:1834.125
before armor and without talents.
Also factor in the damage that Plague Strike itself causes:
511*.50+189 =~445.4
AND also figure in the bonus damage to your Blood Strikes you use to convert the runes:
One Disease: 511*.40+305.6*1.125 = ~573.75
Two Diseases: 511*.40+305.6*1.25 = ~637.5
Now factor in that with the IT glyph you add 10 RP per 20s rotation and with HB you add about 304 base damage to your "beefed" FF applicator.
OH! And I almost forgot to mention the ADDED DOT DAMAGE from Blood Plague, which was buffed in the patch.
In total I feel the added disease with the IT glyph is better. I will rerun this after work as I am typing in my office and semi-rushed.
Hope that helps some though!