The black head is the .28-rr which is supposed to be more powerful. The green head is the spec 2s which is cheaper.

Many will tell you the axial engines arent that good, but I have over 10 gallons on mine and it still has great compression. I just had to tear it down for a new front bearing and carb reseal, but it has been a great engine. Always starts easy and is easy to tune. Its not a real top end screamer, but good for a monster truck IMO.