Rancher:- 3 hatches per day (more chances to hatch a colour)
- Access to your Rancher Shop
** Pets generally take a longer time to sell than items**
Merchant:
- 10% off NPC shop items
- 1 hatch per day
- Access to your Merchant Shop
** Items generally take a longer time to sell than pets**
So if you're looking to make a quick small buck and then compile it to make a bigger sum of TU, I suggest you choose Merchant. If you don't mind waiting to earn a large sum of money, you'd want to choose Rancher.