I prefer the vanilla generation for their bases. It was tricky to figure it out, but if I set them to minimum occurrence and minimum size, they wind up as huge biter cities with significant space in between for travel. As I lower their occurrence rate, it clumps them together without reducing their total numbers, if anything it increases them. At default settings, they're sprinkled across the landscape in tiny hubs and you can't make more than a couple steps without running into aggro range of three or four of them. With minimum occurrence and maximum size, I found I could move around just barely, but most places there was a bit too little room between them. Lowering their size has little impact with each setting, and only at the minimum is there really a lot of room to explore between them. I'd go even lower if I could.
RSO gives way more open space for travel and also seems to fix tree generation, but it sprinkles biter bases randomly all over the map, all with exactly the same size and the same ugly squarish rhomboid shape, no pattern to their spawning behavior and frequently blocking off paths entirely. The only way they don't prevent travel is when there's so few of them that it is sometimes difficult to even find them, and always extremely easy to set up camp so far from them that pollution becomes a non-issue.
In vanilla I was able to set up an imperfect but still very cool spawning system which reliably made paths between biter cities leading from one clearing to the next. I wish RSO could make them generate something like that, only with more space between them. In vanilla it is impossible to get any more space between biters without eliminating them completely.
Here's a screenshot from my game, using vanilla terrain generation (with Alien Biomes by Earendel), without the use of RSO. Biter base settings: occurrence: very low, size: medium
http://i.imgur.com/V4OUAYZ.png
