There's a lot of things that could affect a seedlings height (distance from light, moisture content of soil, strain of seed). I wouldn't worry about one being bigger than the other, especially since they are different strains that probably germinated at different times. Just be happy that both came up and are looking somewhat healthy. I would watch the stretching on that big one. The plant getting bigger doesn't mean that it is growing happily, it means that it is stretching for light. If you want to avoid getting a soft spot in your stem and the whole plant falling over and dying, I would support it with a stake (chopstick or wooden skewer) or just add some dry dirt around the base... enough to give it a little more support. The best thing you can do for it right now is to get it under some proper lighting. Try a 4' fluorescent T8 lamp or a CFL. No matter what light you choose, you need the plants pretty close to the bulb and you need the light to be cool enough to allow that.
