I'm not noticing any ads from the software on my phone. I am still working on replacing the Google stuff though, since it is presumably spying even if it isn't spewing ads.
On a recent iphone setup I didn't notice ads per se, but instead there were constant pitches to buy subscriptions to Apple services. I found that just as bad as ads.
The Moto G's currently come with 3 years of security updates and 1 Android major version update. That is probably good enough. More than 3 years out, the phone hardware probably can't keep up with the increased levels of software bloat. My old phone had 2GB of ram which was fine when I got it, but near crippled now.
I don't even know how much ram my Maemo phones (N900 and later N9) had. They were dog slow though.
Cost is no guide on a crud-free experience.
If you wanna see spyware crud Try buying Chinese phones with Candy Crush and third party Chines apps and games you can't uninstall and show you ads in the menus.
Also factor in how many years of SW updates are you getting on those cheap Motorolas and also the quality of the software.
Hardware bang for the buck ain't everything. If the software is full of ads on those cheap phones or is buggy(like it was on my OnePlus), then your $250 phone ain't worth shit in terms of frustration it's gonna cause you long term.