science or food from trade routes?

I'm curious how other players decide, early in the game, if they want to use a trade route to another civ which will give them science directly, or a food caravan (which will give you science somewhat indirectly). Do you have any definite way of making that decision?

For me, I tend to look at what % increase in science a trade route will give. So, if I am at 15 science, and a trade route will give me +3 science, I will probably take that, as percentage-wise, it gives a good boost. If I'm at 100 science, I would opt for the food caravan, as +3% seems weak.

So I guess if a trade route will boost my current science by at least 15%, I will take that. If not, I will ship food to the capital. And this assumes tradition. Didn't think about liberty.

How do other people decide?

Food will quickly mean more science on top of other stuff.

My rule of thumb is:
-Early gain of science: pick external route
-Long term gain: pick internal route (and provided you have the happiness to sustain growth)

The main use of a science route is early on if you have multiple critical techs to get. For example that petra rush or having to take both composite bows and multiple luxs. If you have all the early techs you need consider food then.

Food caravans are also easier to protect.

Also Cargos are usually a no-brainer => Food (easier to protect too).

My general approach is:
tradition+MP/Immortal- >> food/land to CS for quests sometimes
tradition+Deity >> sometimes first for science, mostly food
liberty >> first for science+gold/cs quests, then production

It is more solid to focus on food, not science early game, cause there is no much techs that boost you production or food (Like 3? Civil service, Chemistry and Fertilzer).

Emperor and below: Food will trump science no matter what. (You should catch up to the AI really quick)

Deity: For the first route, Science is likely to trump food, especially if your capital is inland. (At least until after a few renewals.)

Immortal: If following the Tradition path and both the capital and the other city are coastal, then a food cargo route to your capital will trump science.
For an inland route, careful math needs done for the Tradition path.

Note that the yields from external routes aren't set in stone during the lifetime of the route; they are recalculated every turn as both you and the AI discover techs.

Theres no such thing as being too happy. Get the internal trade route up quick asap and then keep your cities growing until you use up all your happiness. Your libraries will make a lot more science that way.
Workers up soon enough can get your cities some more farms for more food so that could be another way to spend happiness. The point here is to get more science with a library and a larger population. Nc and future science buildings will add more science and wont grow that much since you can make specialists. However, if you find unhappiness soon and cant grow, then going the trade route way would be recommended since unhappiness could make internal trade routes an exception.