They are both similar and in a class of compounds called Growth Hormone Secretagogues (GHS). Hexarelin is also in this class.
These compounds are interchangable in various related studies and you can find more info if you read through DatBtrue's CJC/GHRP thread.
The main differences would be their strengh in GH release as well as their additional stimulation leading to increases in cortisol and prolactin.
Hexarelin is the strongest GHS, GHRP-2 is somewhere in the middle, and GHRP close behind it.
However, GHRP-6 has the least significant impact on Cort and PRL, and is the cheapest compound of the 3 on the market on average.