I assume you mean Be3 and NOT Bd3 as Bd3 would be Illegal.
However, While 1...a5 2.Nb5 might put the Knight on an outpost, what else is it doing? You might have light pressure on d6, but first off, it's hard to add any more pressure on d6. Secondly, when you do take, if somehow you were able to win it, your d3 pawn becomes super weak, and you'll probably drop the pawn back to him eventually.
Also, you have to look at more than just the Knight, but also the residual effects. What happens if Black eventually trades Knights with you, whether it be via the blunt Na7 or something else? What if the Bishop trades itself on b5, so as not to allow Nb5xWherever, but instead, Whatever Takes Knight on b5. How would you capture back? Neither pawn capture is ideal.
However, if you go to d5, and Black ever trades on d5, White can recapture with the e-pawn, and White has the e4 square available as a launch pad for his pieces (such as the bad bishop on g2).
If Black doesn't trade on d5, the Knight controls more key squares around the King, like e7 and f6, instead of being out in la la land controlling a7.
I have a repertoire book that suggests me to play the symmetrical English. Against white not playing for d4 eventually we reach this setup:
So white's plan is to play b4 and I have to stop that.
The book gives as possible a6 to meet b4 with cxb4 axb4 b5! blocking queenside play.
But the mainline is, from the position in the diagram, to play a5.
Continuations in the book are Nd5, Bd2 and Bd3.
Question is, what about Nb5? It's not discussed in the book and it seems to me the most natural move for white, getting the outpost square and putting annoying pressure on d6.
So, is Nb5 not that good for white?