The question was asked so long ago, but writing an answer for future visitors
The question is interesting for opening slight advantage. But I don't understand what did you mean with the following notation? 1. a3(a4)2. b4(b3)
Did you mean in first move if I play a3 then I have to play b4? And if I play a4 then I have to play b3 to support the forwarded pawn? If that's what you meant then you have to keep in mind that at first, you have to take control of the center with pawns, that's why e4, d4, c4 and f4 is most played move (Let's look at an example so that we can see why should we take center with the pawns).
[fen ""]
1. e4 {e4 is top played move even engines like the move as first move. Cause it
opens up diagonal of the Queen and light-squared bishop. d4 is also good but e4
is slightly better than d4.} 1... Nf6 {Black planned to take center with knight
instead of pawn} 2. e5 {huh $2 black now acknowledged the knight is under attack.
And it has only two squares. g8 and d5} 2... Nd5 {Black didn't want to
un-develop their knight, so he pushed} 3. c4 {Look white has got huge center}
3... Nb6 {Nb4 was also possible but that's bad move cause it will be again
kicked by a3} 4. d4 {trying to paralyze the knight} 4... d6 {It opens a place
for the knight and looking for pawn break. In this position top engine move is
f4 obviously. cause white doesn't want to give up the center advantage. If he
plays something like Qe2 then black will just take the e pawn and 5. Qxe5 $2 Nd6
kicks out the queen. White has central space advantage but not as much as
earlier} *
The above defense is called Alekhine defense named after GM Alekhine. But it gives white slightly central-space advantage so it doesn't favor black that much.
[fen ""]
1. d4 {normal opening to take the center} 1... d5 {symmetrical response} 2. c4
{challenging to give the center} 2... Nf6 $6 {not good move} 3. cxd5 Nxd5 4. e4
{White has got center advantage again. So the common move is 2... c6 (Slav
defense) or 2... e6 (Queen's gambit declined)} *
[fen ""]
1. b4 {Polish opening. This doesn't favor white so it's not played that much. It
can be played when a higher rated player is playing lower rated player.} 1... e5
{Just taking center and attacking the b4 pawn} 2. Bb2 {totally giving up the b4
pawn and attacking e5 pawn} 2... Bxb4 {It actually favors black even after Bxe5
cause the bishop can never be removed from a5. and It can used as an attacking
weapon.} 3. Bxe5 {Attacking g7 pawn and followed by Bxh8 so black has to stop it
with the move Nf6.} 3... Nf6 {It's kind of equal and if white do general
development like Nc3 Nf6 e3 or e4 followed by Bishop anywhere. Then there's a
line where the dark square bishop will be trapped. (I actually don't remember it
but it's available in YouTube). Though engine says the position is equal but in
my eyes black has slight advantage cause the black's dark-squared will be great
piece for attacking and it is pinning d pawn so white has two central pawn (e
and d) but he can't use one of his pawn for about 7-8 moves. And rook won't get
a the e file to control so it will have to be on f1 for so long.} *
Kingside or Queenside space advantage (if you have kindside and queenside then that's another case) < Taking center < Attack
I have got another example where pawn supporting pawn is not the best thing (cause attack is greater than anything).
[FEN ""]
1.e4 c6 2. d4 d5 3. f3 dxe4 4. fxe4 e5
In the above position, I was thinking that if I take the e5 pawn then I will get doubled isolated pawn (though isolated pawn is not the bad thing always, but I don't like it more often, in this case 5... Qa4+ so my king be under attack and wide open in the center). Another idea was 5. c3 cause 5... exd4 6. cxd4 for a moment I totally forgot the fact that Qa4+ exists. So I blindly moved that pawn and I lost that game. After the check I thought, I will be manage to artificial castle on the Queenside. But opponent was so higher rated (around 2000) and for the less opening knowledge I lost. But the main line was
[FEN ""]
1.e4 c6 2. d4 d5 3. f3 dxe4 4. fxe4 e5 5. Nf3 {Qa4+ is more dangerous than losing a pawn.} exd4 {you don't have to capture that pawn for the same reason} 6. Bc4 {you developed faster and Ng5 is threatened. And you are ready to castle. Rook will be happy to be on an semi-open file}