The new Beaglebone Green is actually not that different from the original Black. The most notable differences are that they have removed the HDMI port and DC input jack to make room for 2 Grove inputs. The more important difference is that it is significantly cheaper($39 vs ~$60).
The Beaglebone Green is exactly like the Black in all other aspects, and because of that I love it. I personally never used the HDMI port, and the lack of a DC input jack is not a big deal as we all have a ton of USB power supplies these days. The rest of the board is the same and you should refer to my Beaglebone Black review for the details.
The other thing I would like to talk about is the Grove System. It is an interesting modular system that does away with breadboards and jumper wires by introducing a standard 4 wire connector to interface with all peripherals. This is great as it makes your hardware setups a little cleaner, and almost completely gets rid of the chances of human error in wiring. However, I would like to point out that their sensors are quite expensive, and not really of a superior quality.
They have a very good range of sensors available on their website and this was the reason I actually bought the Beaglebone Green. I picked up their starter kit and few more sensors, and I have seen that they are great and much faster to prototype. But in the end, as a hobbyist that buys tons of components a month, their choice is still limited and the vendor lock-in coupled with the high price tag makes me sure that I'm not going to buy anymore.
The Beaglebone Green is a great board, I would recommend it for anyone looking to buy a Beaglebone Black for hobby purposes as it is cheaper and the only thing really missing is the HDMI port.
The Grove system on the other hand is something I don't really recommend, as first and foremost they are expensive, and you will be perpetually shelling out more money to use them with any new microcontrollers you would want to buy, and even if you were ready to shell out the money, there is no guarantee that they would have the correct shield/cape for your MCU.
I do however, recommend the grove system if you are just starting out and don't already have peripherals. It eliminates some sources of error that most beginners face and that would justify the high price tag. I would also recommend it for prototyping new hardware products, as then your budget is less constrained and the modularity allows for easy upgrades and component switching.