Longer is better than taller, especially if when you have a double story cage, the levels are quite short. The space to hope multiple times along a length is more beneficial than just going up to another small level. And as you have said yourself, ramps waste quite a lot of space. My cage is double story, but it’s also 1.8m (5ft 11) long. If you have to room to expand lengthways then go for it.
You could buy a nice long cage, you can also buy NIC style grids to make a cage yourself. Examples: https://www.google.com.au/search?q=nic+rabbit+cage&espv=2&biw=1600&bih=775&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjiopzBpKPMAhVHFZQKHUZcCAQQ_AUIBigB
That way you can custom build it to whatever size you have space for and if you wanted to, down the track you could also add a second level on top of the longer length giving even more space. You can even buy another puppy play pen and build a cage from the panels of that, using some panels for the roof.