I have 2 options to design a list view, the list item is a sentence:
Just list the list items (sentences)
Put a line between each list item
Which is better?
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It depends on how and where you want to use the list. As a best practice for better usability, it is best to represent a list with alternative row colors for each list item - this makes it easy to read, which makes even bigger difference in a list of sentences as user can read each sentence and know without any confusion where it ends and the new one begins.
Lines as separators is a trivial way of using list items, but still prevalent in XHTML based WAP pages since it saves space and size of page.
You should check the ratio of line spacing and the height of the gap between sentences. If you can cluster each sentence and provide nice breathing space between items then you don't need separators. The more important question here is whether each item is selectable and has some signs or buttons, or something like 'disclosure indicators' next to it, then you better use separator lines. And if you use them make them weaker than the text: use pale/dimmer/more neutral color, so they won't distract from the content of each item.
Based on your list view.
If you use more data: Use "header" with bottom line segregation for each rows
If you use only one data type (like sentence): You can use bullets or sentences without line or some padding
If you use multiple data a. Use different color for (even row / odd row) like gmail tabular view b. If you use multiple data within single column (like twitter), just you can use row bottom line.
NOTE: Here, If you post sample data, we will help you better than this.
As the others said, it will help to have an example of the use, but here is my answer.
I'm a believer of less is more... until more is more. That said, I got used to keeping list items separated by white space IF there is only a few items AND it's clear that each is an item on its own. Also, if you know that the list won't grow you can use this approach.. as it will keep the list clean, classy, simple and easy to read. When the list is a long list of items or could potentially grow, using separators is the best thing (either lines, colored rows or other) as it will help the users to quickly spot the rows, and in turn the item that they may want to locate. Personally, I like the use of thin lines as separators as I find them quite unobtrusive to the eye, specially if in a neutral color. Also, if the list includes items that can be selected, remember to make sure the users know which item/s are selected using some visual resource (different coloured row, border, etc.)