MTB or cyclocross bike?

Rigid mtb with drop bars. With fast xc tyres you'll be able to keep up with roadies. At least based on my experience...

The applicable phrase here is 'horses for courses'.

I've just this morning dropped my old mountain bike in at the bike shop, and am the proud owner of a shiny new X bike. Having got used to the speed and position of a road bike, I couldn't face the slow squidgy feel of a mountain bike. On the downhills, sure, it has an advantage but on the flat and the uphills I just ended up annoyed how slow it was. The X bike is lovely by comparison.

CX bikes are fast and great fun if the track is relatively smooth, gravelly or just stony. Once it starts getting rocky and rooty (this is not a scientifically accurate measurement of trail roughness BTW ) then the riding position, rigidity and relatively narrow tyres will start to hinder rather than help. Personally, IMHO I would get a fast, light 29er hardtail. Put up with the slightly slower speed on tarmac but enjoy the increased fun off road. You have a road bike for road so this would mean you have all options covered.

I've had a bit of a MTB renaissance recently - ironically because my cx bike is stuck in the workstand with some technicall issues - and had forgotten how much fun it is having a bike you can batter over all sorts of terrain.

I run an MTB "touring" chainset on my (non racing) CX think that's a 48 36 26 with a 32 at the back pretty easy if slow on steep (25%) hills - old 9speed triple so a bit dated a 10 or 11 speed CX compact and 30 or 32 at rear will give some good hill climbing capacity

CX is great on single track and roots - post says not rocky stuff - that's good because rigid forks, narrow tyres and high stand over make rocky stuff too hard work on the downs and tricky on the ups - at least for me

A bit evangelical but if you like road and singletrack and like to connect them up a CX bike does it and does it very well

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Descents are fine, it's ascents that may be a problem on a CX bike - the lowest gears are nowhere near as low as on an MTB so you will end up hopping off and carrying the bike up hills.

Depends on the climb. There's very little in the Peak District that I can't get up on my CX bike (34-30 lowest gear). You just climb differently - more out of the saddle than sitting there spinning.

I really like my CX, it's pretty much my go-to bike for most "normal" off-road rides, I'd only ever take the MTB if it was involving some properly technical terrain.

Pros are that I'd go further/faster on my CX since it's easy to mix even fairly long sections of road in with it. Cons are that I'd avoid the really technical descents.

Descents are fine, it's ascents that may be a problem on a CX bike - the lowest gears are nowhere near as low as on an MTB so you will end up hopping off and carrying the bike up hills. In a CX race that's exactly what you should do as it's quicker to run up a hill and then remount, but it may get a bit tiresome on a leisure ride (I know that when I'm off-road on the CX there are some routes I simply avoid because I know I won't get up them).

You may well find that a hardtail 29er bike is ideal for you - 29ers roll fast, and they're available with better geometry these days so you shouldn't find manoeuvrability a problem on singletrack. Bear in mind that a CX bike will have 700c wheels which are the same as 29er anyway - 622mm.