Sorry to hear about the gearbox.
Once repaired it should last well over 50,000 miles - maybe 100,000 if you treat it well. No guarantee on the other components of course - suspension could well crop up, you might need another cambelt change and you need to check if the subframe is quietly rusting away. Electrical gremlins could strike at any time as well - its all a lottery, even with a brand new car.
And by 2016 ish, when a Giulia finally comes to the UK it is likely to be a lot more expensive and your 159 could be worth pennies. My 159 1.9 Lusso was £22,500 RRP in 2008 and that wouldn't really get you into a similar spec Giulietta nowadays.
Tough call, but once repaired you need to get some of the investment back - drive it and enjoy it.