We hold onto our Macs for a long time. It used to be that, when we purchased a Mac, we maxed out the processor and added more RAM later. We can do that anymore. Everything seems to be soldered in (to get the slimmest and lightest possible laptop?). Now we max out everything in the machine of our choice. The Mac cannot be upgraded 2 or 3 years later.
One benefit is that you get more for your Mac when you sell it. You also have received better performance from it while you have it.
If the spinning beachball shows up a lot in a 2 year old machine, there will be regrets.
My advice – buy the best MacBook you can reasonably afford. If you don't mind using an external drive to store the bulky stuff (video files, your iTunes folder) you can get by with less on-board storage. That is one way that you can economize. My iTunes folder for the MacBook Air is on an external drive. (Same with my MacMini.)
By the way, we no longer buy the fastest desktop – you can buy a nice used car for what a MacPro costs. And neither of us has a 15" MacBook Pro. My husband is due for a new laptop and will go with a 13" – but will max it out. In the next year or two, I'll do the same.