I have a bantam and have ridden a borrowed fantic on occasion.
The fantic brakes are better and the bantam doesn’t have a reed valve so won’t rev and provide as much power for longer hill climbs. Having said that I ride the bantam on modern trials at clubman level and it does manage very well.
Perhaps the thing which your missing which is what I suffer from and no longer why I have a modern bike is that I can ride most things on the bantam and ride the BMCA british series which gives me best of both. I’ve never really taken to twin shock because the bantam will do it all and enter British events. Classic bikes are also on the up so a British pre 65 will always hold good money long term.
Youll also find that pre 65 events have easy middle and hard routes that are generally easier. Just don’t forget a “competitive” pre 65 will cost a fair bit more.
Rich