I say the Siren looks like a blast to learn on because it appeared to me to be big enough to camp on, the cockpit looks large enough for friends. For me, a big part of sailing is watching the stars come out at night, and waking up on the water.
I purchased a Buccaneer as my first boat not knowing ANYTHING. That was a go-fast plaining hull with a lot of sail. In retrospect something a little more forgiving would have been better. I can see you loving your boat for many years.
bljones comments seem to confirm your choice from someone who knows the boat.
I say...GO FOR IT! Whats to lose? Its just money, even if its all the money you have (easy for me to say). The up side is you acquire a healthy life long passion.
Boating can be enjoyed your whole life. I raised two kids sailing. It can turn into a family activity as you get older, unlike other vices.
What does your girlfriend think?