Are those expressions equivalent, or which one should be preferred?
I should finish this work sometimes next week
I should finish this work sometime next week
Although they look similar and are both adverbs, they are not equivalent.
As you can see, replacing it, it still makes sense.
If you try to replace it now, it won't make sense:
This last one should be used for example to say:
sometime means "at a single unspecified point in time."
sometimes means "at various unspecified points in time."
When you say "I should finish this work..." you're talking about specific work, and it's only logical that said work can only become finished at one point in time, so only "sometime" makes sense.