Ultimately, it is up to you and your values and budget.
Here is my perspective for you compare/contrast against.
For me, if I bought the LT, I would add the $375 cruse connectivity package (cruise is a must or I'd loose my license on the interstate - "I can't drive 55"... or 65 or 75).
I would also want the Fog Lamp/Wheels package to get the 16" wheels and fog lamps for $300. Personally, I'd be happy with 16" wheels, but would pay the price to move away from the 15" wheels (the fog lamps are nice, but I wouldn't pay for them).
A sunroof is not worth $850 to me and I would be very happy with the LT 1.4 after adding the $675 in options mentioned above at a MSRP of $17,865.
However, the LTZ 1.4 costs $18,695 or $830 more. That is a typical price for adding leather seats and a great price for leather plus heat!
At this point, this becomes a very hard decision for me. I live in GA, so heated seats are not a big deal and I can make an argument for the breath-ability of cloth. The leatherette should stay cleaner, and if it is as good as what my Jetta has, it is very durable. Also, one of the magazines commented on how the leather steering wheel was nicer than the Corvettes.
I went ahead with the LTZ because my daughter is a big fan of the leather, the auto delay lights are handy, and I think it is the only way to get steering wheel audio controls. But from my perspective it is a splurge that I rationalize as being easily offset by buying a car for less than $20,000. I want it to be a "luxury" econobox and think this makes the Sonic the closest you can get in today's market.