Can the differences of an 18MP and 24MP camera be spotted? I have a 500d, old, but still decent for the use I give to it...
Given a large enough print size, the difference in image quality will make itself apparent. The question then becomes, what is that print size?
The graphic in this answer leads us to believe that the difference is a print size of ~11x17 and ~13x20. And honestly, that's not taking interpolation into it. It is possible to increase an images size up to a point - but one has to take the viewing media and distance into account to calculate that.
I'm going to just go out on a limb and say that, no, for most consumers, there will be no noticeable difference.
However, given more pixels, you gain the ability to crop into your photo and still have enough pixels for a decently sized print.
And if we're talking about newer technology, and not just pixel count, there are gains in image processing technology that lessen the noise at any given ISO.
There are many reasons one might upgrade cameras - but more pixels for the sake of more pixels is not one of them...Unless you like making the day of the marketing team at (Insert Camera Company Here).