I haven't tried either. I know, I suck.
I'll say though, that unless you're extremely sensitive to caffeine, the Diet Coke is nearly completely psychological. It only has about 30mg of caffeine, whereas coffee has 100mg. Caffeine pills contain about 200mg. Energy drinks vary, but if my memory serves me right they contain about 70mg (and a lot of sugar and other things to actually make it "work").
In other words, quit downing the diet coke. I can't personally vouch for Power Drive/Spike/Surge as I've never tried them. If I were you I might try Spike, for the mental clarity you seem to be seeking, in addition to a pre workout drink - and one that isn't toxic (Diet Coke).