I completely agree with you. Dealers only offer discounts on petrol version and not on diesel version(they have booking for it).
I had faced this same issue when I went to check for WagonR Duo when it was launched in hyderabad. I had been to a showroom with a friend of mine. They behaved in the same way. They were acting as if it was our necessity. Just because of this he went to take Santro. Thankfully it was a good car. I had the same experience from the showroom I was going to take. The first execuitive was not offering a single discount and was acting as if it was my need. I contacted another executive, I said I am dealing with so and so person. He didn't budge much. A friend of mine forwarded my number to a third person. He was jovial, treated me like a customer and gave me a good discount.
In short what I can say is that you can blame dealers/executives. Dealers take you for granted when there is only one outlet. It wont be just if you blame Maruti for that. They have kept on calling me 4-5 times after the delivery of the car. They enquired about how was my experience etc etc to the minutest of the detail.
'How come ? Here they plainly refused to give any saying " arre bhayi yeh gadi to pehle se hi booking par hai, discount kaise/kyun denge ?"' - I got the same reply when I asked about diesel. 
BTW this case is common for all dealers/executives/states.
Just try the way I did. Maybe it'll help you. 
EDIT:
Posted by Daljit Singh
What if I said (ahem..) I want all these 
Actually, the more I read about both the cars, the more confused I became !

Everyones becomes so...........
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