Just bought a Ti 1.9Jtdm, I have done a lot of searching on here and also the web in general prior to the purchase.
The 2.4 gets less flack on here than the 1.9 engine and most people on here would tell you to steer clear of the 1.9Jtdm, but if your not going to thrash it, and your not interested in reaching warp speed at the blink of an eye, then the 1.9 would be fine and cheaper.
There are reported issues with the 1.9, as with any car, so take it as you want really. If your a worrier and a glass half empty kind of person, don't buy an Alfa full stop.
One thing of note I found, there are far more people on the web who haven't had problems than the noisier few who have, so take your pick and enjoy the car!