The Ecotec is a much simpler engine, built of cast iron, with no complex mechanical components, proven over many, many years in hundreds of thousands of cars. It is amazingly economical for an old nail and utterly reliable if treated properly (ie serviced regularly and not abused).
The Alloytec that you are considering was a new engine in the VZ, not as proven as the Ecotec and has been shown to have its problems, particularly with timing chains in some engines and various electronic sender units in the early models. It is less torquey than the Ecotec, meaning it requires a bit more throttle to give its best performance. Repairs are much more complex and even simple servicing such as changing spark plugs takes a good deal more time and effort than the Ecotec. The thermostat is a bitch to replace compared to the Ecotec.
The Alloytec should give better fuel consumption and performance as it is a more refined and powerful engine than the Ecotec. It really comes down to how well the car you buy has been treated and how well you look after it.
I agree that 10k will put you in a really schmick high spec VY or VZ, and even get you into a lower spec VE. However, the VE would be an early one and they had their share of probs, mainly electronics, as well as some of the early Alloytec issues, such as crap quality oil pressure senders, which leak and set off the engine service light.