That depends on how good you are with pillz and how strong your cards are in the first place. Cards with normally low power such as Bunny, Acid DC, Josh, Klaus, Nobrocybix and Akiko get more from attack + 8 while stronger cards like Tanaereva, Hikiyousan, Wee Lee, Bodenpower and Platoona are benefitted more by power + 2.
Depends on the card. If it is a lower power card then attack +8 is very useful if it is a higher power card then power +2 is more useful. Think this way power is 1 then +8 attck is like using 8 pillz on that card free...
Attack+8 if you use 1 to 3 pills (including the "free" one); Power+2 if you use 5 or more pills; equally if you use exactly 4 pills (4x2=. Come on, this is really simple mathematic and is - in one way or another - asked for every week.