The truth is that there is no real difference other than the colours that they are painted. The diameter and height of both is so similar I can't tell the difference just from looking at them.
This is great because they can be used with other products from both brands! I'll get to that soon but the first thing to think about is what colours you're wanting.
Now if you're wanting the colours of the peg people to match the colours of the Grimm's Rainbow perfectly I'd go for the Grimm's Friends as the 12 Friends match each arch of the rainbow exactly (pictured below).
The Grimm's friends also match perfectly with the Grimm's other products such as the Sorting Board, Building Boards, Semi Circles, Small Balls etc. Therefore if matching perfectly is up your alley they are the sure choice.
However the Grapat Nins (pictured above) are also rainbow coloured but they are different shades. We started our collection with the Grapat Carla Set and added Mates (little cups) and I have to say that they have been played with everyday, especially the rings and coins.
Another thing that people don't realise is that you can just buy half a Grapat Carla Set called the Nins, Rings and Coins. These are available in rainbow colours and warm colours and together both sets make a Grapat Carla Set. This is something that you can do if your budget doesn't allow for the full set up front.
A similar option that Grimm's sells is the 7 Friends in Bowls. We don't personally own it in our collection as we own the Grapat Carla set with Mates which as you can see is quite similar. With both the peg people fit loosely inside the bowl which is great for young children to place them in and out and stack the bowls up high.
As always if you have any more questions never hesitate to contact us, we're always happy to help!
UPDATE: In 2020 Grimms updated their 12 Rainbow people to make them more inclusive by using a mix of painted and natural wood "faces". The photos above are all using the older style Grimms friends with all white painted "faces". They also released the Grimm's 12 Rainbow Friends Cherrywood, they all have natural "faces" with visible woodgrain which are absolutely stunning.