I'm using straight flow. Straight to the point answer:
Pros:
- More responsive high-rev
- Power don't deep after 5500rpm for stock engine + stock TD04
- Less turbo lag
Cons:
- Loose low-end torque.
- Car felt slightly hesitant on traffic jam stop and go. (before turbo kicks in)
If you're planning for stage 2 mods and above, then go for straight flow. Else s-flow works perfectly for mild mods.