Let me give it a shot.
SDI, or semi direct injection, injects relatively low pressure fuel into the intake tract of the engine, and this occurs while the exhaust port may still be uncovered.
An Etec engine, on the other hand, is essentially BRP's term for direct injection. This uses a relatively high pressure fuel injection system and injects the fuel directly into the cylinder after the exhaust is covered. Because the fuel pressure is higher, and the timing window is shorter than for SDI, it is more difficult to achieve.
The advantages of pursuing direct injection though, if done well, should be fewer emissions, especially hydrocarbons, increased power, better fuel mileage and better 'drivability'.
In addition, at least with BRP, they have also reduced oil consumption further than what they had already achieved with SDI.
I'm sure others can provide even more details. Snowtrax has a decent YouTube video explaining Etec btw.
