Short answer: Both, but perfer positional rankings with tiers
Longer answer: I tend to use a positional rankings sheet with tiers, then rely on my knowledge and research coming into the draft to know values across the positions. I think by doing it this way and having positional tiers as your main cheatsheet, it can be a double edged sword if you don't have a decent knowledge of values across positions.
The double edged sword is that you can identify and make quicker decisions during postional runs, which is a positive. However, you can also be more easitly influenced to take the last player in a particular tier over a better player if you concentrate too much on it.
This is why it is always a good idea to keep an overall rankings/ADP cheatsheet handy to double-check and make sure you are not reaching.
I like knowing who is left at the positions I need to fill in the middle rounds and having those tiers right there makes it easy to identify which positions I need to fill first to maximize value. So having an overall player rankings list is a little tougher to read. Of course if you have the ability to have your computer right there, you can do a 'sort' on Excel based on position and filter your rankings rather quickly.