Hypothetically If you had a choice of riding the same bike on an ITT with a choice of either 27+ or 29er with a 2.1 what would you do?
The ITT in question is the upcoming Braunton 150. There is only a negligible weight penalty to go plus.

27.5+ and rigid vs 29 with suspension?

Same bike. Fargo. No suss.

In 2017 I rode the B150 on 2.35 650b with a 120mm fork.

This year I'll be on a 29er with 2.2 tyres. I usually have this bike set up rigid but for the B150 I'll be putting a 100mm fork on it. Given the dry weather bits that are usually boggy might be baked hard so plus would be my choice.

Obviously if this proves to be the wrong choice you can't blame me

I've never ridden a plus bike, what's it like on tarmac? I seem to remember some steep twisty descents which you might need to take care on? But then I'll be taking it easy on them as well as I've slid along the tarmac once this year and that was enough

The only MTB I've had for a few years until recently was a 29+ They take a bit of getting used to on road but ride fine. For my purposes anyway. I don't intend to take any risks on the road anyway.
Haven't been able to get a suss fork for the Fargo yet. I rode 100km with the 2.1's last week and found it ok but the going wasn't overly rough, quite a lot of hardpack and gravel.
Tending towards the plus as I don't fancy having my head kicked in for (I hope) 24 hours.

I've been trying to get the right balance for my Stan (the Sonder ). Stans a 650b (think thats same thing as plus right)...

I started with a shiny Fox fork in 130mm guise and a Trav Prong. The Fox got sold on without ever smelling the frame and the Prong stayed.

3.0 front and rear were what I started with (Vittoria Bombolonis I hear you ask) and it was nice running such low pressures but it was all a bit vague and (for want of a better word) Rodney Trotterish...

But I needed some more oomph and ground feel so put my bulldozer type 2.5 inch (WTB Convict) tyre up back with the 3.0 remaining up front. The ride suddnely became more responsive and exciting but still a bit dull and floaty upfront...

Aaaanyway. To cut it short. I'm currently on a 2.5 (actual is 2.5) rear and 2.5front (actually closer to a 2.35 or 2.4). Have a 2.8 specialized butcher Grid or that sort to go on front and will keep the 2.5 at the back or swap to the 2.35 (actual size) from front to back. I think that would be the perfect Stan(ce) with some decent brakes.

So... if I didnt already say it. I vote the wider tyres/27.5plus thingy....

I think it depends on the terrain - I think you'd get through fine either way, but you're probably going to come out of it in better condition with a plus tyre setup. I wish I'd had my bike in it's current 29x3" guise for the HT550 rather than the 29x2.35" I did use - my left hand might have retained its grip strength rather than the wobbly mess it was for a couple of months afterwards!

^^ was that rigid or suspension?

as one of my bikes can do either i swap around - broadly 29 er faster 650b+comfier but draggier

Depends on the terrain and/or whether you want speed or comfort

Rigid.
Currently flip flopping towards 29x2.1.
I think it's worth a try just to see how it feels. It's not like I'm going to be troubling the scorers but might help on my sub 24hr quest.