I went with the synthesis ending. At the time I didn't see anything inherently wrong with merging synthetics and organics, after all, Shepherd is already basically a cyborg and it's worked pretty well so far. My only real misgiving was how in the HELL that whole thing is supposed to work "Merge synthetics and organics to create a new DNA?" how exactly does the giant green laser DO that? Also, at that level isn't organic material nearly indistinguishable from inorganic material and AAAAARRRGGGHH!!! In hindsight, a lot of people have pointed out that I'm imposing a radical change on everyone in existence against their will, which is pretty fucked up when you think about it, but honestly the thought never occurred to me, so I guess that makes me a fascist. Great, now all organic life can fuse with their cell phones.
In subsequent playthroughs, I went for red. At first I was afraid to pick that option because the line between what the magic red beam would and wouldn't kill seemed vague, and I was afraid that it might kill all the quarians who had integrated Geth programs into their suits and I didn't want to hurt Tali.
