First, though, here's the most relevant part from the area terrain rules (p. 91 of the BBB):
"Models in area terrain receive a 5+ cover save, regardless of whether or not they are 25% obscured. Models that Go to Ground in area terrain receive +2 to their cover save, rather than +1".
So, to the arguments.
The area terrain part of a ruin only grants a 5+ cover save
Okay, so I'll concede this one.
The rules for ruins (p. 98) clearly say to treat the base as area terrain. Nothing else. So, the upper levels are not area terrain, and the base is not itself a ruin. Just standing in the open on the base would therefore give a 5+ save, while being obscured by the ruin itself (walls and what-not) would give a 4+.
So no more standing in front of the wall, on the part of the base that sticks out from under the ruin itself. and claiming you're in the ruin. Tough luck.
If you're on the base, you cannot claim the 4+ save for the ruin
See the 'regardless' up there in the long quote? Innocent enough, innit?
This argument states that, since you get a 5+ save regardless of whether you're obscured or not, you get a 5+ save, and nothing else.
And if you could only have one save, it would be correct. But see, 40k works on the notion that you can have more than one save. In fact, I will admit that it's a bit more fuzzy on whether you can in fact have more than one cover save, but it is crystal on the fact that if you could, you use the best one.
I will not start digging too far into the semantics of the word 'receive' (we did that with 4th edition Tactical Marines, and it was nowhere near as much fun as some people seemed to think) but nowhere in the meaning of that word is the phrase 'and lose everything you had before' included. You receive one cover save from the ruin, one from the area terrain (and the receiving of this one doesn't require being obscured), and if you're behind a Grot, two units of Terminators and a spool of razorwire, you also receive another three 5+ saves and one 6+ one. In the end, it doesn't matter; you'll only use the best one.
I say you get to use the ruin if you're hidden by it.
If you Go to Ground on the base of a ruin, the best save you can get is 3+
Because if you Go to Ground in area terrain, you add +2 to 5+, and if you do the same behind a ruin, you add +1 to 4+.
Except the rules for area terrain say you get the +2 to your cover save, not to the cover save from the area terrain. So if you have a 4+ cover save for some reason (say, oh, there's a big wall between you and the enemy) you would add +2 to that one, for 2+.
And, if you had (for some reason - say a Warpsmith made your forest less habitable) a 6+ cover save as your only friend to begin with, you'd end up with a 4+ save after hugging the ground.
And that, in the words of Forrest Gump, is all I have to say about that.
Ta, and DFTBA.