A lower head tube will give you a lower aero position so +1 for the Sempre. For the IM and C100 rides you will want something 'comfortable'. This does not necessarily exclude the Sempre as comfortable is a many faceted thing at 90 miles, regardless of bike. It is about fit - especially if you want to run 26.2 miles afterwards without being dicked for the mincing prize*
I've looked at the geo too - other than the length of the headtube there really isn't much in it.
Sempre it is then! Do they come with men's chainsets, or just compacts 
*Even though gladys would win anyhow.