The 98 is way heavier than the 99.
If you're happy with the 99 get another. If you're happy with the 99, then no its not worth the extra money at all.

An inbetweener would be the HW95. Same action as the 98 so essentially the same rifle without the window dressing and weight penalty.

The 57's a handy little rifle. Similar cocking linkage to the 99 with similar issues relating to that.
The marmite issue is down to the pop up breech. Earlier ones with plastic breech blocks had issues. Later ones are metal. Its just down to whether you find the pop up breech ok. Personally I liked it.
Thin barrel and can be noisy when running on song as they have a fair crack especially in .177. Ugly front sight unit is probably its worst feature. If they made the forend like a 97 I have one again.