If you're willing to fork out the cash for TM, I'd say to go Classic Army. No hesitation there for me. About the same price (maybe a bit more), all metal (best stock metal bodies of any AEG IMO), reinforced internals (including the mechbox shell itself) that CAN handle a stiff spring without any other foolery required. The all metal contruction won't flex in the slightest, unlike TM and other plastic guns.
As for compatibility concerns, there are no compatibility issues with CA guns. I've been running my CA M15 rifle (now with a Guarder C7 body) and it's been fantastic. It has minimal internal upgrades, shoots 385, and has sick range and accuracy.
My custom C8A2 and MP5 are also built from a lot of CA parts (externally) and they're both incredibly solid and look great. Although they're really not CA, so it doesn't apply in terms of the internals which are all custom.