It depends on what sorts of positions you prefer. If you go c4, you don't allow your opponent to play ..d5 in response as easily, unlike you would with starting d4. If you go d4 first you can play openings like Queen's Gambit.
C4 or D4. .???


That's me copying and pasting from the Game Explorer database here on chess.com. The number of master-level games played with said opening move, followed by the win percentage for white, followed by the drawing percentage, followed by the win percentage for black.
From a practical perspective, neither move really offers white any better or worse game and the difference is mostly a matter of what kind of positions you prefer playing (although many 1.c4 lines do transpose into 1.d4 systems).

As you can guess from the fact that both are often played at GM level, they are equally good (and just as good as, say, 1.e4 and 1.Nf3). That said, 1.c4 may lead to some rather refined positional lines that are probably not the easiest thing in the world to play for the average club player. From a practical point of view I think most sub-2000 players would be more confortable playing either 1.e4 or aggressive 1.d4 lines, since the strategic ideas tend to be a bit more straightforward, but if you like 1.c4 better there's no reason to stop. you just shouldn't think that something like a "best" first move can exist.

If your opponent is great with d4 openings, and you rule with c4, then go with c4 and avoid lines that transpose. If you suck at c4 openings and black is great at them, go with d4 if you're better.
im not exactly sure im just guessing, i think when you start c4 you rule out the sharp counterattacks, but thats just a guess

If you play the Sicilian with black then 1.c4 might be better for you as this allows reversed Sicilian.
I mainly play 1.e4 but sometimes choose 1.c4, it avoids the Slav for example because white can use the Reti setup. Also, after 1.c4 f5 white has the possibility to play d3 instead of d4 and then black lacks the move Ne4 which is typical for the Dutch. This works great against the stonewall, white plays d3 and is then able to break the wall with e4. Another thing is 1.c4 Nf6 2.Nc3 g6 - now white can play 3.e4 which avoids the Grunfeld (some players feel better against KID)
Thus in terms of flexibility, 1.c4 gives white more possibilities in my opinion because the d pawn has not committed yet.
However if you feel good in QGD and Slav structures then you should rather start the game with 1.d4 because it avoids e5.

I like C4 more because it's more explosive.
lemme guess... is this ur favorite song:
http://www.youtube.com/watch?v=Vysgv7qVYTo
Very interested in what is better as white