Both and also what is the best way to evaluate and determine what a good rental investment or flip opportunity is?
Many investors typically will have varying cost of funds (how they finance the investment), different investment horizon (2, 5, 10, 15 years etc.), variations in local market conditions where they invest and expected minimum rates of returns that vary.
How you look at any investment as an investor is personal and affected by your particular investment goals and objectives.
The closest you would get to a one size fits all approach is by utilizing the "70% rule" for flipping. With rentals you may hear of the 1% or 2% rule. These aren't always applicable or feasible in every market but is a start for many noobs.
The IRR, Net Present Value etc. are also tools often used to analyze any type of investment -- be it a flip or rental.
You will get a million answers, but BP is the greatest place to start. Flipping and Buy and Hold are totally different animals. I would start with the Free How to Guides under "Learn" at the top of this page!