There's some debate as we're moving into new superservers that we should only build level 10s, that level 5 NPCs are for sissys.

I farm both, and find that lvl 5s are too important to avoid.

Farming 5s is free and easy.
(1) I lose nothing. It's free food, free xp for low (or high) heroes.
(2) A ballista team is dirt-cheap to make and maintain, marching costs are negligible.
(3) If I get attacked while my ballista teams are out, my defense is not impacted.

Farming 10s only is not sustainable.
S'pose you lose 7.5k worth of archers per hit on a lvl10. Actual costs may be higher or lower depending on your hero, what types of fodder you send, etc., but I think this is close, and the conclusion doesn't change if you have more or less.
7.5k of archers takes ~5-6 hours to make (again depends on your hero, # of barracks, etc.)
7.5k of archers takes 2.25M food, 2.25M iron, and 2.625M lumber. You get 19M food, 0.6M lumber/iron/stone, and 1.5M gold.
It costs approx 600k food to march, depending on the distance.
So the net from a lvl 10 hit:
+ approx 16M food, 0.6M rock, 1.5M gold
- 1.65M iron, -2M lumber

Also, while my troops are out, my defense IS affected, depending how many teams I send from each town, I WILL have losses I need to be online to heal, and depending on my archer-barracks production, I may not be able to make as many troops as I lose if I'm planning to farm multiple 10s twice per day.

Would I trade 3.5M of lumber/iron for 16M food? yes of course
Do I enjoy leveling up high attack heroes? yes of course

But I'm not going to forego the bread-n-butter of level 5s.

I only (only?) play Evony a couple of hours, a couple of times per day. With a good field of lvl5s, I can launch my attacks and come back hours later to find millions of resources gathered with no cost, no troops to heal, no vulnerability to my defenses while I was afk.