Personally, I'd prefer the T-Virus. That virus produced things far more scarier than what the Las Plagas injection did, it just wasn't scary.
But if somehow, they can implement the two, and still make it feel like true survival horror, then bring it on. There is no way that the T-Virus and the Las Plagas injection should both be included separately in the same game, I just think it wouldn't work, and could confuse us fans on how they both caught up with each other