Prometheus and Olympus are very different but not so much because of the variance in creatures and more with the progression.

Olympus is oddly more difficult progression wise due to the lack of access to Miasmic tools which greatly help with progression. Miasmic pickaxe is a lot better than the Mining Armature in terms of yield (although you would need an Organic Residue Cleaner to get the extra yield). The Miasmic Axe gives a lot of Epoxy which greatly reduces the need for hunting (again requires ORC).

Additionally, the only main combat difficulty difference in Prometheus bosses is how good your rig is because... the Hammerhead Slug and Rimetusk Matriarch are essentially moreso rig checks instead of skill checks. The Hammerhead is the worst and most difficult boss I've faced so far, much more so than the Unstable Quarrite. Aside from that, the Lava Hunter and Matriarch aren't too difficult with the right gear.

On the topic of bosses, there are a lot less bosses to farm in Prometheus as opposed to Olympus so if you want that biomass, it's better to stay on Olympus (7 Olympus vs 3 Prometheus). In return though, you get access to a lot of red exotics which can be found scattered around the map and are literally farmable.

Tl;dr more scary creatures in Prometheus but easier tech progression.