i would say if you get hot in the ergo you're going to need something made out of solarveil or solarweave or something. if you don't plan on using them in the water, try getting one where just the panel area is made of that material and the straps are twill or something - it'll allow the air to flow.
also for shorter periods of wearing, you might like a ring sling or pouch - my favorite summertime ring sling is a single layer of linen or a linen blend in a light color

but there's only so much you can do in terms of the carrier to keep cool when you've got a warm little body against you. babywearing can be a sweaty experience in the warm weather, to some extent there's no way around that. that's one of the reasons my DH likes our frame pack, the baby's not right up against you. personally that messes with my center of gravity, but it might be something to consider too.