It's definitely a close race, but I think Taxi edges it out:
- Different and more varied shots to pick up passengers vs. shooting the same deadly targets.
- Combo-able ramps in Taxi vs. repeatable ramps in Diner.
- Multiball in Diner can be cheesed by never stirring the cup.
- Spinner shot in Diner is both important and difficult to shoot. Unfortunately, it can be cheesed quite easily with the Today's Special hole. This makes multiball and Dine Time Jackpot much easier to achieve compared to Taxi's equivalents.
Honestly, it may come down to personal taste, but that's what makes Taxi the superior to me. Diner is certainly no slouch; I'd play it on location any day, if there were ever any around me.