I thought I'd see if there are any new contestants for an omniwheel, and there's this one I haven't seen before that showed up at the top of the list:

In the listing, there were 3 similar wheels that looked really cool, and for some reason, the darker it became the more it could carry.

It's twice the price I'd like to pay for a wheel, but I get 2 wheels so...

Anyway, as it turns out, the different colours are because of the different hardness levels:

Considering that the other wheels I was going to go for were heavier and only held 40kg, any one of these seems like a win. Looking at research such as [this] and [this], it seems that the harder the wheel, the easier it is to slide across surfaces, particularly hard ones. The outdoors is predominantly hard surfaces, though it's rough and hard. I may want some vibration damping found in the 60A and 85A options. 

I think the 85A option has the best balance between aesthetics, sliding ability, durability and potentially vibration damping, though I can see why the seller recommends the 60A. The additional dampening and low noise could be beneficial.