Just wondering.

If you have to choose between elf and catapult, which unit will you choose?

Although it seems like comparing a level 1 unit with a level 3 unit, but somehow elf seems to be a more powerful
range unit since it can get double shot, is more mobile and it range attack can increase with increase in morale.

But I have not really trained catapult to high level.
So I am not too sure how powerful a catapult can be.

What do you guys suggest?

In my opinion it is good to have catapult:

-good bonus to siege(and it is quite important if you want to capture the capital city before powerful hero of enemy will arrive)
-good range and powerful shots (to be honest, catapults and elephants helped me in fight with dragon, I haven't lost any of my unit, but I wouldn't done it without catapults, which were strong enough to weak the dragon before melee fight)

And you can have the catapults without making alliance.

I prefer good variety of "magic-shooters" and "normal shooters", but I can't help to choose anything else than shamans on 1-st level. Of course if you want, you can have 1 or 2 elves, but in my opinion it is better to spend this "place" in your army for magic shooters.

I see.

Anyway, is it possible for catapults or any mechanical units to get medals?

So far my catapult has not manged to get any medals even though I let it kill most of the units.
I was trying to get the sniper medals for my catapult.

They can get medals.
http://www.gog.com/forum/eador_genesis/award_medal_details_request_to_russian_speaking_friends

For medal of sniper you need to deal a lot of damage and score at least 1 kill.
Finishing blow is not enough, 1 kill equals to 5 damage.
Buffing it with bless, magic weapon and astral energy could help.