New bike or fix old???

Hi there
I have a specialized hardrock bike($600) 2 years old with about 6000 miles
done on it. I fell off it when the chain broke at some traffic lights and it
needs to be fixed up, estimated to be about $270
This bike continuosly seems to break gear cables (about 5 times since new)
and spokes so I guess I need better wheels (it hasnt broken one for a while
but did it a bit after I first got it)
It needs new derailers (they got real bent up), cranks (the front gears??),
chain and a new cassette (it has a new hub in it).
What would you do??? The bike store guy offered $120 trade in for another
$600 bike (a hybrid meridia 8500 for $610) or a mongoose thunderball for
$1200 (with $200 trade in so it makes it $1000).
Any opinions on either of these bikes?? The moongoose is probably way more
bike then what I need.
I weigh about 100 kg all up including water
I do commuting rides (9 km ride to work 1 way ) mostly but a full on
mountain bike looks cooler.....
$200 doesnt seem too bad on a $600 bike that needs a lot of work. Anyone
have any ideas on how much the frame is worth?
Stephen