Which is better? Old or New?

You are looking at the wrong factors.

If in anything you want to look at the ongoing potential. Given roughly the same position and location .. both can be good investments. The lower liability on the half block may mean lower rates and overall expenses such as heating and garden are minimised. The investment potential on the land component of the older block depends on what the councils will allow you to do with that land for subdivision.

So the correct weighing up of the investment should be in the expected ongoing potential weighed up against existing expenses and liabilities. Eventually with property you reach a point where the unit of land cant be divided up or extended any further. At that point you end up riding inflation to make your ongoing gains.

The two things you must do when purchasing property is forget sentiment and just judge the property on its current existing potential. What might have been a fabulous investment in 1980 may not be a great buy in 2012. And vice versa.

It would be a worthwhile exercise comparing the council rates and such.  I recently had the opportunity to see the council rates, body corporate fees and such for a new "house" for 2011 and also 2009.  I was able to see how rapidly the bills had risen and it was disturbing.  Basically what had occurred is that a developer had bought the street, and turned the whole street into a body corporate.  The council rates were wayyyy higher than those of an older house on a plot of land of twice the size just around the corner (a location of equal stature) and the body corporate fees had gone up astronomically.  The body corporate is covering the upkeep of the naturestrips, the street lighting (that's right – it's not council's problem), the roads and pavements, and buiding insurance.  A couple of years ago I observed that a new subdivision (ie new suburb) near my home has been developed as a body corporate.  The whole suburb is a body corporate.   It's poor residents are even forking out for the local leisure centre… whether they use it or not.  Be wary of the holding costs in such new subdivisions.  I would imagine that it's the way of the future – state government squirming out of maintaining the streetscape by having entire suburbs declared body corporates responsible for the upkeep of their own roads and street lighting (and remember street lighting attracts an electricity bill, and that electricity goes up each year).  In this regard I'm very wary of new areas and prefer older areas.

I always look at older houses only if it has a potential to either subdivide or renovate and add more value. Make sure you check how much it will cost for repairs/maintenance as well. 
As your a FHO you'll have to live in the house for the specified time until you can rent it out. In this time if you get the older house you can do reno's and claim depreciation over time AFTER its rented out. Don't think you can not claim repairs after while your living in the house though. Older houses still have depreciation for some things but not near what you'll get for a new house.

The 26K for the new house sounds nice but if the rent or land value doesn't increase then you only have rent and depreciation as income. New houses have the most depreciation at the start and get lower and lower, how long do you plan to have the property?

Check difference in insurance costs for the new and old house as they could differ.
Have you checked with the council to see if you can subdivide the old property now or in the future and if the market wants new houses on sub divided properties? Maybe buy the old house subdivide and sell the land?

The difference in rent is bugger all.