The advantages droids have over soldiers are that droids don't need food and their attack/defense power is greater than soliders. Also droids are much cheaper with respect to gc, since droids also need iron to build.

But the ratio for how many soldiers to droids in a battle can be calculated using their attack and defense.

Soldiers:

Ground attack: 5
Ground defence: 5

Droids:

Ground attack: 6
Ground defence: 7

yep, i'm wrong

how much food does a soldier consume per tick? 1?

Although droids take up a fat chunk of your race points and their fair share of Iron - they are definitely worth taking ...  for attackers.

But for a banker?

I bank with no droids in my custom CF banker race.
The reasoning behind this is; You are not going to be doing that many attacks (if you have a decent active attacker with you). For mid round defence, instead of droids and ground defence, station a load of Figs and max on air defence, (or use portal if you dont fear getting Figrunned).

Later in the round when you could do with ground defence too, to back up your lasers and Figs, spam build a load of soldiers. At that stage everyone usually has mills of spare food floating about late round. Give the iron you saved to your attacker, or OB build more CF's
Do i make sense?