Biorbs are definitely not worth the money. They appeal to people because they're round and look more decorative to people, but they're not good for fish, and they cost way more than a usual rectangular tank does.
There's a reason the hobby has mainly stuck with rectangular glass boxes, and it's not just about ease of making them. It's because they work out the best for the fish we tend to keep.
The downsides to biorbs include that you need biorb products for them. So you'll need to spend even more than just the initial cost of the tank. They also scratch easily when being cleaned, spoiling your view of the fish. Also imagine it from the fishes' point of view. When they look out, everything they see will be distorted.
Live plants are also better for the long term health of the tank, and fish prefer them - it's much harder to grow live plants in a biorb, and sharp plastic plants can rip tails, especially not recommended for bettas, with their long, delicate trailing fins, or guppies with their large and easily damaged tails.
Bettas are solitary fish, some are more territiorial and aggressive than others, so some of the more laid back ones have been kept with other fish in the past... but guppies are probably the worst to live with a male betta, since those long tails can make the betta think it's another betta in his territory, and they're very capable of killing other fish. That's why they were bred and gambled on as fighting fish, after all.
They also need different water conditions, with betta being soft water fish, while guppies need hard water. If you look at your local water companies website and give us the number and measurement used for GH, KH and calcium, someone here will no doubt be able to say whether your water is soft, middling or hard - in the fishkeeping sense, so then you could choose fish that match the water you use!
You could probably keep a reasonable number of guppies in a 60L biorb without a betta. But I'd recommend getting a simple 60L usual tank with filter and heater from Gumtree or similar, and giving it a go with that first, rather than waste the money on a biorb that will really limit your options.
But, I admit I'm biased against biorbs. I hate those things for all the above reasons, and more! But we'd be happy to try to help you with your hobby plans, whether you decide to go ahead with a biorb or not.