Blue Yeti would be better quality for streaming. Modmic is a good microphone, but is really a microphone for voice chat. Yeti is a better quality microphone more aimed for recording purposes.
Another thing to take into consideration; is the quality of the onboard audio the laptop has, which will be used if getting a Modmic. It will likely be better on a gaming laptop than it will on a budget one for example. The Yeti, being USB, has it's own sound processing and doesn't use onoard audio.
If you're going to use the microphone for both streaming and in game voice chat, then the Modmic might fare better for voice chat. Being close to mouth, will likely stand a better chance of not picking up unwanted keyboard and mouse noise. That said; people do use the Yeti for game streaming. There is mostly likely a way to position and set it up, so keyboard and mouse noise is minimised. If it picked up everything, I doubt streamers would use it.