Speeds vary based on usage by neighbors on the same node but I have still have better succes with cable over DSL.  DSL top data rates can be too slow for HD streaming.  The upload speed is also to slow to allow Sling TV.
I have Comcast and a 400GB monthly limit.  Their limit ranges from 200 to 400 GB depending on your tier.  At 400 GB, I don't have to worry.  If I was at 200GB I would with high Netflix usage.  I learned that if I use Spotify on my computer it can drive usage up becuase even if I listen to a 10 second clip when building a playlist, the Spotify app downloads the entire song into the cache.
I can check my usage on Comcast website and there isn't a fee for going over, they just send you an email if you bust the limit a couple months in a row.  If you are at a lower tier, they probably try to get you to upgrade.  I'm at the highest tier so I don't know what they would do.  I've never hit the limit.