Over time I've developed a priority for booking flights using points and miles. There are definitely exceptions to these guidelines, but I've found the following priority to work for me.
1) Any Expiring Non-renewable Credits/Miles/Points
The only examples I have in mind are Southwest LUV vouchers and unused travel funds from rebooking Southwest flights at cheaper prices. If you cancel, or rebook at a lower price, a Southwest flight originally purchased with cash, you will receive non-transferrable travel funds good for one year from the travel date. These travel funds cannot be renewed by booking another flight with the funds and then cancelling said new flight.
2) Any Points Tied to a Credit Card You Want to Cancel
This should also generally have the condition that you do not have another credit card in the same family to which you can transfer these points. For example, if you have both the Chase Sapphire Reserve (CSR) and the Chase Ink Preferred, which both earn Ultimate Rewards (UR) points, even if you wish to cancel the CSR, it should not be priority two on this list. However, if you only had the CSR, and no other cards that earn UR points, then the CSR should be priority two.
Furthermore, it should also be clarified that I'm not talking about airline specific cards such as the Delta Amex. The Delta miles you earn from this card are not at all tied to the credit card once the statement closes. On the other hand, if you have any UR points on your CSR when you close it, you will lose all of them.
3) Points Whose Redemption Value is Maximized at Certain Price Points
The Merrill+ Visa from Bank of America offers redemptions of 25,000 points for a flight up to $500. For every dollar the flight is over $500, it is an extra 100 points. Hence, if the flight is near $500, the Merrill+ card should be favored.
4) Miles/Points vs Cash
This is where it becomes more ambiguous. Some people will say never to use airline miles unless you get 1.5 cents per point (cpp, as in the cash price of the flight divided by the cost in points), or 2 cpp, or whatever figure they pull out of a hat. And in some situations it definitely makes sense to use cash and defer using airline miles for a later redemption. However, you have to keep a couple principles in mind.
- Miles/points almost never increase in value; they continually lose value while cash can be invested
- If you never use your miles/points because their "redemption value" is too low, then in reality your points are worthless
In general, if you're earning miles faster than you can spend them, then you should use miles instead of cash.
If you decide to use miles, you should generally prioritize using miles already in your frequent flyer accounts first before transferring points from credit card points such as Ultimate Rewards (Chase), Membership Rewards (Amex), or Thank You Points (Citi). If you can preserve the flexiblity of UR/MR/TYP, you generally should. A possible exception would be if, for example, you already have 100k Delta Skymiles, and it would cost you 80k Skymiles or 60k Ultimate Rewards (UR) for the same itinerary---then I would look at how many UR you have, and your ability to earn more Skymiles vs UR.
To help you decide which miles to use, you should use Award Hacker (there's also Award Ace but it has been down for months and I don't know if it will ever come back). You put in your desired itinerary and it will tell you how much it costs in miles with essentially all airlines of interest to travel hackers.
For each airline it will also tell you which programs you can transfer points from (UR, MR, TYP). You can also filter your results based on which airline miles or transferrable credit card points you have.
It is important to note that neither Award Hacker nor Award Ace will check the availability of your desired flight at these prices. It merely checks the cheapest possible flight according to the published award charts.
There's a similar site for hotels: Award Mapper. Put in a location and it will show you the various hotels and how much they cost in points. Unlike Award Hacker though, it will not tell you the transfer partners for each hotel brand.