At a very high level:

Spring is written in c++, scripted in Lua, and released under GPL2.

OpenRTS is jvm based and released under an MIT license.

Each open source project carry with it some model AAA game legacy. Starcraft 2 is for OpenRTS what Total Annihilation is for Spring, Age of Empire for 0ad, C&C for OpenRA.

For now, OpenRTS is specialized in micromanagement on cliffed/tiled terrains. All the game content is accessible from outside the code, using provided editors, XML and scripted langage. It is also Android (almost) ready. Those are the main differences with Spring.