I actually had both engines - 2006 Jetta with the 150HP 2.5/manual and a 2015 Jetta 2.0/auto 6 speed.
The 2.5 was much better all around - Had it ~88k miles and never an issue. Like others said it is thirsty, but it has grunt and actually I liked the "uneven" noise it made. Torque for days but ran out of breath in higher revs. The 2008+ 170 HP is the version to get as they corrected some timing chain issues, was a much better performer at higher revs and was a bit smoother.
The 2.0 - if you are ready to keep it on the boil all the time to keep up with city traffic than by all means go for it. If this is for driving around in rural areas I see no issue though unless you encounter passing maneuvers. The 2.0 even with the 6 speed automatic just wasn't working out for me in Atlanta and I felt like I had it running up to redline nearly constantly, it was an emergency purchase when I couldn't get my '12 Fiat 500 (f'ing FCA) to pass emission and was not in the $$$ position to secure something beyond the base Jetta S, one year later I was able to ditch it with only 8k miles on the clock. It was just tiring to me to be just running it ragged to safely merge onto the freeway, I don't know whether to feel bad for the person who got it because I rode it so hard or they should be happy it was well broken in - knowing the 2.0 I probably gave it a good break in. It legit is a tank of an engine.
2.0
+ Slightly better on fuel than 2.5. Best I saw in my '16 2.0/auto was 33 hwy on a long 10 hour trip to Florida which was exactly the highway rating. The 2.5 gets slightly worse but has +55 HP and much more torque to back it up.
+ Reliable as [censored] since it is a 30+ year old engine design.
+ Sounds pretty good for a 115HP engine....on the downside the noises it makes don't backup the actual movement. It sounds pretty mean and has a nice growl.......pedestrians will get a good hear of the growl because it will take you forever to move past them.
- Still has a timing belt that needs replacement.
- Low power - yeah it will squirt off the line quickly but runs out of breath as soon as its off the line. I think VW tuned the electronic throttle to be like an on/off switch. I never felt much more forward momentum no matter how much further I pushed the pedal. I kid, I kid.
- You will be replacing spark plug wires (yes I said wires) on a modern car.
2.5
+ Way more power than the 2.0, enough to motivate a Jetta nicely.
+ I personally like the gruff sound the 2.5 makes but your results may vary. Its more of a groan instead of the 4 cylinder buzz.....its unique.
+ Timing chain - have not heard of many issues with the timing chains on the 170HP version, there were known issues on the older 5th generation 150 HP version.
- More thirsty - The best I did in my old 2.5 was 29 which was 1 MPG above the EPA hwy rating. Well worth the fuel economy hit.
Honestly I would say try to find an older well maintained 2.5 in the same body style, the engine was well past its early '05-'07 teething issues and the timing belt replacement alone on the 2.0 would probably eat up any fuel savings you achieved going 2.0 vs 2.5.
