Take this with a grain of salt, as I haven't screwed around with, or really looked at, the Evo ecu, but here's my impression:
The Evo ecu is probably cheaper to get/set-up, but requires more work, where as ECMlink is more expensive, but relatively easy to get up and running.
They both have a lot of tuning support, but ECMLink probably has more DSM/Galant specific support than the Evo.
I'd imagine v3 ECMLink has more functionality than the Evo, but that's not really based on anything.
So it really seems to depend on what you actually need, capability wise. And how much support you'll need. If the Evo will meet those needs, I'd say go for it, otherwise ECMlink is proven.