A 91 running at reduced throttle settings will use less fuel and make a whole lot less noise than a 70 running at full throttle. If you can learn throttle control, the ONLY reason NOT to use a bigger engine would be extra weight.

In your case though, a 91 four stroke weighs less than the 61 two stroke that the Tiger 60 was designed for. Putting a 70 in that airplane would be a decision you would surely regret.

Jim