I have sailed both: TIGER and Inter 18.
Their preformance is only in minor things different. The more full hull around the bow areas on the INTER lets it @Slam@ a bit more than the TIGER.
Their boat speed is about the same. Maybe the SPI cut is better on the TIGER. But that is about it.

HOWEVER:
Talking about quality features, esp. the rudder design on the TIGER is a lot more durable that's why it is my favorite.
The rest is about equal.
TIGER advantage:
You may sail in a TIGER one design class as well as F18.

Any other thoughts?

Fritz