Having owned a Sportivo, and now an Astra SRiT. I do think they both have their own strenghts. The Sportivo was a fun car because of the noise and people didnt expect it from a Corolla.
The Astra however has the best quality interior and overall finish out of the 2. As for power the Sportivo has 141kW and the Astra SRiT has 147kW. So almost identicle, but the Astra has it over the Sportivo because the power is there down low, but you have to wait for the power in the Sportivo. As for looks, i much prefer the look of the Astra to the Sportivo.
In the end it comes down to what you prefer Klara, and what you are willing to pay.
When modifying the Astra performance parts are becoming more readily available. I know that Walkinshaw Performance in Melbourne have parts for the SRiT, including exhaust, CAI and lowering springs. And there is always Ebay.... Check out www.opelaus.com if you have any questions. Here are some pics i have of both my cars, just to help you choose :D