I think you shouldn't build sightstone first, getting a philosopher first is a must for supports i'd say. I usually even get a kage's for more gold right after philo, and not buying sightstone until the laning phase is over. Imo getting a few wards every time you back is far more cost effective for the laning phase than getting sightstone early on.
I think it depends if the kind of support you are playing.
If I pick taric or leona, I usually go for sightstone because it saves me gold + makes me more tanky, which I need to do my job properly.
If I pick sona sonna, for example, I go straight to phylo for the extra mana reg + gp10, because I know I should not be caught (and if I am, I'm pretty much screwed anyways) and that my job is to poke and spam my abilities as much as possible, and sightstone wont help me do that.
So, at the end, it is about what you need to do your job better. There are times I rush phylo with taric because my adc y pushing like a champ and the only thing I need to do is heal him when needed so he can win trades, and ward river. I dont still dont need the tankyness.
-If my lane is being constantly pushed up to not being able to ward river often and I usually have wards when I back, I get a philostone, where you might as well use the wards you bought from the beginning of the game. The philostone helps to sustain in lane a bit better as you're going to be harassed a lot under tower usually if your lane gets pushed a lot. As I'm not using a lot of wards because of being pushed up, I don't get the sightstone first.
-However, if the lane are equally pushed, and I DO ward river/tribush/lanebush often, I often get that sightstone before philostone. It may be costly upfront, but it provides (I believe) 180 health (worth 475 gold like ruby crystal), the other 475 gold you pay is basically an 8.33 gp10that provides vision if you assume you place 2 wards every 3 minutes (which you would if you got this item in the above situation). With philostone you pay an additional 550 gold for +6mp5, +7hp6, and a +5gp10 It takes the placement of 6 ghostwards to make up the cost of the other 450 gold you spent (6*75=450). The earlier you get sightstone, the more you save on wards on the long run. Just more efficient (gold saved is the same as gold earned). But if I don't have enough for the sightstone upright, I get the philostone as gp10 is better than no gp10.
That's from a gold standpoint if you are trying to get ahead of the other support.
Thus usually I tend to get faerie charm>sightstone>philostone>rubysightstone
or faerie charm>philostone>sightstone>ruby sightstone.
With the new sightstone change a regular first back can get you a ruby crystal and some wards, if with that money you decide to complete your philostone instead you may want to figure out how much money you will have from that philostone on your second or third back. I don't know specific numbers, however most of the time that money that you earned from the philostone will cover the cost of a single pink ward.
If the lane is very passive and you aren't taking a ton of burst in fights, then the regen from the philo may benefit more than the health from the rubycrystal/sightstone. Play style can also factor into what you want, if you start with regen bead you can always make a locket or aegis with it later.
Regardless of what you do build, you should put your money towards 1-3 wards if you don't have a completed sightstone, pink wards are always helpful.
I hate how expensive Sightstone is, it's the most overpowered item ever, and it's sooo expensive! I can jungle, but I'd best take a Giants belt instead of a Sight stone, cuz unless my mid lane knows to use that mini-map and to help me gank properly, I just traded my ability to fight with team sight.
It really depends on how the lane is going for me. Winning, getting my exp, able to stay out longer and pick up most of the gold for a sitestone if not all? I'll go for sightstone. If the lane is being rough, making me stand under turret. Get no assists. Frequent returns I'll go philo stone, or kages depending on teh support. Or, if their jungler is a big trouble maker like Hecarim who can run past wards super quick, I'll try for sightstone and still buy wards to plant them deep and all over.
It also depends a lil on who the support is. If it's a support I want a lil more expensive enabling set of items. Philo. If it's a support I want to build tanky, I actually go for sight stone quick. It's got nice health and the site are just nice.