This is the same situation as I was in before I bought my Leon. The majority of cars I've owned have been Vauxhalls and I suppose there was quite a bit of brand loyalty. I test drove the 1.9cdti sri 150 before finding the Leon and the ride is good for everyday comutting and long motorway journeys though it gets boring and tiresome after a while. I found there to be a suprising lack of feedback and I was never sure how far away from the limit I was when throwing it around twisties and country roads. The interior is bland and boring, particularly the centre console; it looks like there should be something there with all the empty space.
The Fiat sourced engine is a strong contender and almost every bit as good as the VAG unit but I found that the turbo doesn't give the same oomph as I get in the Leon. The performance feels similar between the 2 but the Astra doesn't feel as quick. Looks wise, I think the Astra (Sport Hatch) wins hands down but I fear that it could look dated soon. Also regarding the looks, when driving around, look in your rear view mirror, they look very similar to a Vectra... I think Vauxhalls designers have become very lazy by making all their cars look the same.
Astras are common too, I don't think there's a street anywhere that doesn't have an Astra sitting in it somewhere. With there being so many around, parts are relatively cheap and easily available. This is what put me off the most. I don't want a car that everyone else has.
IMO the Leon handles better, is more involving and the power delivery feels sportier. There aren't as many about so you get some exclusivity and many jealous looks at traffic lights from Astra drivers. The Leon is more spacious (sp?) and still looks gorgeous being a 5 door. Granted it's more expensive than the Astra but they hold their value alot better come trading in time. Brand snobbery also comes in to play... "My car is better than yours because it's more expensive" type of thing....
It totally depends on what you want and what your driving style is. If you want comfortable (and somewhat boring compared to the Leon) cruising, get the Astra. If you want to be progressive and feel involved, get the Leon.
Just my 2p.