Craig
Not read all of the above so sorry if I duplicate.
I arce on the road and train through the winter on a singlespeed hardtail mtb. I love it. I climb basically everything the geared boys climb. Good strength training. I wouldn’t be without it.
But then it’s my only mountain bike, whereas you’ve got a geared mtb.
I recently bought the new Genesis Day Alfine 11.
I like it a lot. It’s not light! What it’s best for is a sory of on-road / off road training. So I train just like I do on the road, riding through the prettiest areas I can find, riding a few hills and maybe chatting to a ride buddy – but I do it all off road.
I have ridden the crosser on the surrey singletrack and you can definitely do it but it is nothing like as much fun as on the mountain bike.
Both of the bikes you’re talking about are sort of an extra, if you’ve already got a geared mtb. And both are good.
Cross bike: soft trails and bridleways for endurance and tempo rides.
Singlespeed: low maintenance, high fun trail riding.