• So your offensive stats with GA + Sorc +triforce are: 80ap & 65ad & 15mpen ( & 20% cdr).
• With deathcap: 200*1.35=270ap & 65 ad & 15mpen
• With voidstaff: 160ap & 65 ad & 35% +15 mpen
• Note that i am ignoring all masteried/runes but thunderlords decree (result will havr an error about 5%)

So the single scaling sources without resistances:

• The active of triforce can be ignored as it is not influenced by deathcap or void staff
• passive healing: Deathcap: 60 +0.665 + 0.45270 = 220.5; Voidstaff: 60+ 0.665 +0.45160 = 171
• 2nd strike passive: Deathcap: 100+0.565 + 0.65270 = 308; Voidstaff: 100+0.565 +0.65160 = 236.5
• Thunderlords Decree: Deathcap: 180+ 0.1270 + 0.365 = 226.5; Voidstaff: 180+0.1160 + 0.365 = 215.5
• Q: Deathcap: (projectile) 115+0.4270 + (proc) 145+0.5270 = 503; Voidstaff: 115+0.4160 +145+0.5160 = 404
• E: Deathcap: 190+0.865+0.6270 = 404 (physical); Voidstaff: 190+0.865+0.6160 = 338(physical)
• R: (fcuk 6.23) Deathcap: 150+0.25270 = *217.5; Voidstaff: 150+0.25160 = *190
• Hextech Gunblade active: Deathcap: 250+0.3270 = *331; Voidstaff: 250+0.3160 = *298;

So now overall at. Lv18 (each skill/active once+3 times ult) :

• Deathcap: Healing=220.5, magic damage=2021, physical damage=404
• Voidstaff: Healing=171, magic damage=1724, physical damage=338

Considering Resistances:

• squishy (50mres, 100 armor): Deathcap: effectiveMres= 50-15=35, True damage=0.742021+0.5404=1699; Voidstaff: effectiveMres= 500.65-15=17.5, True damage=0.817240.5338=*1636, *.
• offtank (100mres, 150armor): Deathcap: effectiveMres= 100-15=85, true damage: 1254; Voidstaff: effectiveMres=1000.65-15=50, true damage= *1284**.
• tank (200mres, 300armor): Deathcap: effectiveMres= 185, true damage: 810; Voidstaff: effectiveMres=2000.65-15=115, true damage= *886**

Now you have the numbers. I'd prefer the rabadons, as it heals more and is stronger vs my targets (squishies, monsters and mininons). Now with double edges sword or merciless mastery my damage is increased in both cases.