Well I am currently a merchant, which means I get 1 hatch a day, the NPC shops give me a 10% discount, and I get a shop to sell items in.
When you get your hands on some good items, merchant shops are pretty good for making tu.
Ranchers get to hatch three times a day, have a 10% better chance to hatch a good color (not natural), and get a shop to sell pets in.
You can make a good amount of tu if you have lucky hatches that give good colors, if you happen to snag really good names (real names or real words), or if you sell your hatches (you can get 100k-150k each for them).
I usually recommend for people who are new to be Ranchers, because you are not likely to have very good items at first.
Another thing is, is that you can change your class four times for free, so that is pretty nice if you decide you don't like the one you picked.