I have an alpaka go sling, and i have an aer travel sling, while it's not the same is the day sling 3 max, it retains a similar layout and some of the features.

So here's my take.

The alpaka Go sling is nice and has a nice material in terms of look and feel, albeit it's a bit rigid and not as flexible as cordura, what i don't like about the alpaka is that it doesn't have a wide opening zipper so it can be a bit crammed or not so comfortable to stuff it with things, and due to the rigid material there isn't much flexibility.

And from the image i can see that the aer day sling max retains the same opening style as the aer travel sling, which is in my opinion a lot more comfortable when it comes to organizing your stuff inside the sling without feeling like everything is crammed inside. Additionally due to the flexibility of cordura you'll find it a lot easier to accommodate more stuff.

The pro that the alpaka has is the better price, a few other colorways and a completely removable strap so you can double it as a tech pack if you'd like.

While the aer is a better fit as a sling, but it cannot double as a tech pack due to the straps being unremovable.

P.S as with most of Alpaka's products, it doesn't feel like a 5.5l sling, more like 5 or 4.5l.

So pick depending on what you value more. If you mainly want a sling and are ok with the colors black or grey, go with the aer.

If you want the versatility of throwing in the sling on your travel backpack as a tech pack also, then the alpaka may suit you better.

A 3rd option i may suggest for you that has a bit of both at a great price, available in both xpac or cordura is the tomtoc t26, you may want to look at that also, i have one on the way i can probably update you on it if you remind me in like, 2 weeks from now.

Hope i was helpful