I do a lot of mixed terrain rides these days and – just to state the obvious – you need a bike that can do the road bits and the offroad bits.
Gravel bikes err on the side of the road bits as they are/have:
– mid-width drop bars
– steeper steering
– shorter wheelbase and stiffer frame
– 40mm-ish small-knob tyres
whereas a rigid 29er is better on the offroad bits, cos:
– wider 700mm+ bars
– slacker/longer geo
– more supple frame/bars
– wider small-knob tyres (2.1 – 2.25)
Both weight the same, if you build a decent 29er.
So it’s all about where you want to make the compromise. Rigid 29ers are compromised on the road sections, whereas gravel bikes are compromised offroad.
For me, my rigid 29er* is the way to go, as I prefer the compromise to be on the road bits. I can still ride 60 miles on it. A mate has a gravel bike and he’s significantly slower on the offroad sections we tackle.
* sub 9kg Scott Scale, Exotic rigid carbon fork, 1×11, WTB Nine Line/Schwalbe Thunderburt tyres, carbon bars/post/cranks/etc.