It depends a lot on the engine if high compression /turboed etc the super would be the sensible option but as the recommendation for in my case (3ltr v6) is the 95, the only real benefit would be getting double shell points when buying vpower maybe cleaner internals too, 98 will probably allow it to pull smoother at lower revs.
My other car however it specs 98 and when running on 95 which it will do so happily, the down side is that it does about 7-8 mpg less on 95.
Unless doing the same journey day in day out in same traffic and weather conditions getting useful figures of economy/performance is wishful thinking at best.
The latest version of vpower (nitro+) in terms of economy has plummeted compared the the previous version so jiggling with formulations does not suit every end user.
