Hello!
Thank you for asking, I'm honored
I did pretty well with my Continental Classics 700x25, then at 3/4 of the long route I had to switch on the run for a couple of Vittoria Zaffiro Graphene 700x25, since i tore one of my continental clinchers apart
I had just one puncture in my whole race, on the Continentals.
I think somewhere between 700x25-28 is pretty much good, depending on how much comfort you plan to get!
The wider the comfier but that said, the wider it is, I find it a bit sluggish too..
My choice is clincher since they're easier to service in case of punctures (you can be pretty sure you will get one, at some point).
Don't put your trust on the mechanics at the checkpoints to have a lot of tubulars, they always run out of them!
I'll be in Gaiole too in October! This time I plan to bring my bike fitted with a couple of Challenge Paris-Roubaix, about 700x26/7, they just arrived I just have to install them
I can't wait to try them since they're an "open tubular", so the servicing benefits of clinchers with the durability of tubulars!
Hope it helps!
--Alessandro