I'm pretty new to this site, been looking through for advice on which 159 to go for.
From what i can find on here it does seem the 2.0 is the better engine but i can't find anyone that's owned both.
You are not looking hard enough
Welcome to AO. I have a 2.0 JTDm SW for about 2 months now. I love it. Previously I had 3 1.9 JTDm 159s and a 2.4 as well. The 2.0 reminds me of the 2.4 - lots of power in 2nd gear, after 1500rpm, below that not a lot - the 1.9 was a little more linear.
Economy between the 1.9 and 2.0 is approx the same - 40-41mpg for me. My previous cars were Lussos my new one a Ti, so the larger tyres will drop economy a little. There is someone on here with a 2.0 JTDm Lusso getting mid-40's easily however.
The 2.0 is smoother, quieter and less prone to the dreaded regen than the 1.9. My insurance increased slightly due to me moving up to the Ti from a Lusso and also because of the nature of my policy - if I had been able to do the update on line the price change would have been minimal but because I had to phone them about something they charged me an additional cost - they'll be dumped in December
Road tax is indeed less because of less emissions. Overall this is the best 159 I have owned - it is better screwed than all the others I have had and the engine is lively enough for me but still economical - I can get over 500 miles on a tank; 580 the best, 500 the worst.