Annual sales- other than Black Friday and Cyber Monday, they seem to constantly have some model or series on sale from time to time, more discount if they have a larger inventory and surplus from time to time (but the last few years, there were fewer surpluses with the chip shortage, larger demand from work from home units, and China shutting down their factories due to Covid quarantines. It is getting better though.
As stated each have a bit of a niche. XPS some are smaller screen usually and convertible between standard screen/tablet mode and for those that are road warriors not wanting to carry a heavier unit. The Precision is basically their workstation line, not really super portable, desktop replacement. The Latitude is their main line of business class in many screen sizes and some ultrabook styles also... Vostro is their small business line- a step above their full consumer line, but not as good usually as the Latitude series. Avoid the Inspiron which is their cheapest and consumer level with the lower end screens and cheapest cases that can break just looking at them wrong...
Decide- screen size you want (13 or smaller for light weight, etc.) 14 or 15.6 inch screen, or desktop replacement of 16 or 17 inch and not very portable. Touch screen model or tablet mode important? And then the things like do you REALLY still need an optical drive for DVD/CD or want a lighter thinner ultrabook series.
https://www.dell.com/en-us/shop/dell-laptops/sc/laptops