They're basically the same. It means, "Hold onto your responsibilities until the bitter end because your word is your bond."
If there's some slight difference in nuance, then the second sentence would be more officious. If two people, a boss and his or her subordinate were talking, it would be kind of an unfriendly admonishment of the worker's poor performance. If two people know one another as friends, then maybe they would say 'kara'. It would still be said in a strict tone, though. So, just the setting: friendly vs business setting.
You could say
以上は with friends, but if you use that then the atmosphere would become very serious. If you used
からには, it's not serious. You know, you might say it to a child who played a video game but didn't clean, and he didn't do the cleaning part. So, it still sounds like you're talking down, a little, if you say