First 700g, ask yourself this;

"Do I still have a ward?"
"Are they clearing my wards?"
"Is the jungler particularly dangerous to us?"

If YNN or YYN, take Philo.

If NYY or NNY, take Sightstone.

As a rough rough guideline.

I always start flask + wards & rush sightstone. The flask gives me all the sustain I need, it's worth several minutes of regen from a philo except you can get it all at once and refill when you need to. Early game can be a bit hard if you aren't careful not to drop wards too early (or late...), but most junglers don't seem to gank as early as they used to and I find that I have a bit of time to ward river.

I think it's ~7 minutes that I need to make it to for sightstone. Of course, if there's trouble I'll be sure to buy more wards before that to tide us over, but most of the time I can stretch the initial wards out until I have gold for sightstone. If you get an early assist, you can usually get a faerie charm first back, too, and it's easy from there.

Granted, I'm Janna so boots can wait for a long time.

Ken sums it up best. If you need the regen, go philo, otherwise go sightstone.