I'd say both too. If you had to choose between them (as there is never the perfect cage usually!) I'd go with floorspace. Width of the cage is as important as length. It allows for a lot of enrichment and toys and tunnels and levels. It's nice if you can give them deep substrate as well, but as Flower Fairy says you can section off an area to have it deeper (if it's a cage you can put cardboard or something inside the bars so the substrate doesn't fall out in the deep area).
I wouldn't say running was the greatest priority - they run in their wheels
They need interest and texture and stimulation in a cage, hidey places, and a set up that's safe if they monkey bar across the roof and fall off, so they can't fall too far or land on anything hard or sharp.
Did you have a cage in mind? I went round the houses trying to find the ideal cage and came back to the one I first looked at! The Barney cage from Zooplus. It's a good size, lots of floorspace (wider than most at 54cm), a good price at under £50 and has 7mm bar spacing so the substrate doesn't fall out
Well hardly any falls out.
The only way to have really deep substrate with a Uk cage is either a glass tank (extremely expensive) or a diy cage. As for running - you can have a roof run in a cage as well as the floor space. This is the set up I had for our last Syrian - plenty of substrate, shelves and levels and a tube across the roof and sputnik linking them, so he could get from one side to the other with different routes. All that substrate on the floor didn't fall out
I had just set up the cage and made a big mess.