I’ve purchased and have been using a number of different helmets in the past year between road and Trail/XC mountain biking.
On the road I really like the Poc Ventral Spin and the Giro Aether. Both are quite aero, relatively light and vent well. I’ve worn them on very hot days with no issues and done long days in the saddle riding centuries and climbing Haleakala. I find them quite comparable. I think either would fit your needs really well. If you shop around you can get some discounts.
I picked up a Trek Specter with Wavecel to use for XC mountain biking. I haven’t had a chance to ride with it yet but hopefully will get out tomorrow. Looking at it, I agree with the concerns @Jonathan raised about it possibly being hot, so we’ll see how it performs once the summer months hit.
Finally, I just picked up the Poc Ventral Air Spin. This helmet is similar to the Ventral Spin but with more slots cut into it. Poc claims it is still aero on only at high speeds is there much of a difference between it and the Ventral. I figured it would be a great option for hot XC days and possible for endurance races like Leadville. I held off on the more pricey XXX as I bought the aforementioned Giro and Poc helmets recently and waiting to hear how people like the XXX on the road in the heat.
The one thing missing is really good aero data. I hope we get some good independent testing this year to better assess the relative tradeoffs between the different helmets in terms of weight, cooling and being aero.