I prepare elaborate spreadsheets with rankings of all models based on factors such as price, accuracy, reliability, capacity, appearance, fun factor, and many others. Then I compare that spreadsheet to a second one that analyzes the gaps in my collection. I carefully analyze all of the variables and create a list of the next 5 guns to buy.
Then, and here's the truth, I'll run across something that strikes my fancy and buy it, even though it was nowhere on my (imaginary) list. Honestly, for the past 5 years a .357 4" revolver has been number 1 on my mental list of what to buy next. Since then, I've bought 5-10 guns and still don't have a .357...