I would say cacti because it sells for more and also to me it seems to grow faster. Just my opinion its up to you to decide to either melons or cacti. :D
1. Have 1 million Mana
2. 1 Billion dollars
3. 5 dubs of Magma
4. Be able to have a Magma farm in your plot.
5. Be able to choose 10 really good enchantments for Armour Tools etc.