Hi Ali,
The HAB DVD will show you how you can make adjustments to your HAB in hot weather - the inventor lives in Byron Bay, so they have to think about these things! I found it fine on hot days (we had a hot summer and it started early, so I got to use it a lot before I got my Ergo at the start of Dec).
For a new baby, an Ergo will not be any cooler to use, as a newborn requires the insert to cushion them better in the sling. If you had all the money in the world, I would say to get both, HAB for when baby is up to about 4 or 5 months old and then the Ergo for the extra versatility of positions. If you're only going to get one, I would say the Ergo - it will take you further (therefore, longer) and it is completely adjustable, so you don't have to buy a certain size to fit both you and DH. I had the luxury of borrowing a HAB until I got my Ergo, so if you know someone who has one, that's the way to go!
As you know, my HAB served me extremely well. Not having used the Ergo for a newborn, all I can go off is my Booby Club friends who used it for newborns and they loved it.
But, back to the hot weather thing - both are very doable, have loads of scope for air circulation for you and the cherub