A standard duna multy might be a good buy just in case a mini duna turns out not to be big enough and your hamster wants more space. It's not expensice and not a bad size. A cage the size of the hamster heaven is good too. If they are tiny then the bar spacing could be an issue on the hamster heaven. You could always mesh one but meshing is no mean feat!
I have the Savic Mickey 2XL for our robo (same as the hamster heaven but 7mm bar spacing) but it's expensive. (About £80).
You can always fill up a larger cage with lots of hides and digging areas. It's a good idea to have a platform or shelf as well. They often end up nesting under a shelf.
What is your other cage? Personally I like 100cm cages and fill them up with things - but you may not have space for that if you have another cage aready (the Barney was mentioned above and it has 7mm bar spacing - but it's also quite tall so could be fiddly to set up.
This is the standard duna multy - 71cm x 46 cm
https://www.portonaquapet.co.uk/shop...a-Multy-M36261
There is also a maxi duna multy (which I nearly got but have my Savic Mickey 2XL already) but that is also expensive
https://www.portonaquapet.co.uk/shop...a-Multy-M58956
I actually moved our robo from a Kerry Terrarium (similar size to the standard duna multy) to the SAvic Mickey 2XL because I felt it would be easier with taming/interaction.
https://www.amazon.co.uk/Savic-Micke...vic+Mickey+2XL
Another option is to start them in a smaller cage for taming and then upgrade but that means getting another cage!
I have also seen the Alexander cage (7mm bar spacing) set up for dwarf hamsters - it is also tall like the Barney (a bit taller) but because it comes with three shelves it's easy to make another full level by using the ladder as a joining strip across the middle and filling the hole with another piece of wood or stiff card and just leaving enough space for a small ramp up the side (with something over that ramp so they can't fall down it).
Alexander Small Pet Cage | Available at zooplus!
I would go with what you have space for and depending on the kind of set ups you like. Natural set ups with larger items like cork logs need more space and it's a lot of enrichment for a hamster, but they can be happy with something less large providing there's enough for them to enjoy in it.
Personally I love the Savic Mickey 2XL so if it's not too expensive for the budget I think it's a good buy. Not too big, not too small and has a big front opening door and 7mm bar spacing. It is also quite a low cage (like the hamster heaven) so not the risk of falls. If I had the space I'd get an Alexander cage for our robo and make a full level.
They can be quite difficult to handle and tame and get out of the cage though! The large door on the savic mickey can make it easier (but you have to be on hand to lift the door up quickly if they come towards the front so they can't make a run for it). Likewise the duna multy's have a good sized door on top.
It could also be an option to get a mini duna if you want them to be tame, as Fluffagrams mentioned, possibly as a starter cage and see how it goes. For Syrians I always think it's best to go as large as you can. I know less about dwarf hamsters and opinion varies on the size they need. For robos some people say they are very active and need a lot of space.
Our Robo isn't tame and I regret putting him in a glass tank as that made taming hard (top entrance and quite tall plus no interaction through bars really). I see more of him in the Savic Mickey 2XL and he is happy with the space. A smaller cage like the mini duna for taming could be an option but you might want to upgrade later.