Is it worth having more megapixels over a better processor?

Welcome to the forum.

The 1500D (or T7 in North America) is the lowest of the entry-level model of Canon DSLR offerings. It was released in 2018.

Its features are either oversimplified or omitted when compared to models of the next level up, like the 650D (or T4i). Most obviously, it lacks a touch-sensitive fully-articulated screen.

I am not saying the 1500D is a “bad” camera. Many of its users enjoy a cheap and simple camera for casual shooting while walking or travelling.

The 650D, on the other hand, is a “very old” camera, released in 2012.

If you value the touch-sensitive fully-articulated screen, and want features more refined than those in the 1500D, I would recommend looking at a recent model, like 750D (T6i, 2015) or 800D (T7i, 2018).

Regarding sensors and processors, it would be difficult to tell the difference between images from an 18MP sensor and those from a 24MP sensor, when they are viewed on computer screens, or reduced in size to be shared on social media. Whereas, a later-version processor, like DIGIC 7, offers many enhancements in focusing and image rendition, including better high-ISO performance. There is not a big gap between DIGIC 4+ and DIGIC 5.

If budget is a major deciding factor, I would go with the 650D.

A suggestion: Consider the Canon 200D (SL2, 2017).

I agree.  You said everything I would have said.

I might add that the Canon T7 (1500D) does not have an internal dust sensor removal system.  If it were me, I would save up for a Canon SL2 (200D) or Canon T7i (800D) camera if the budget was tight, even if it took a few more months of waiting.  I would even use an old Canon T2i (550D) or Canon T3i (600D) camera before I would even consider a Canon T7 (1500D) camera.

That Canon dust removal system is important to me.  A few times I have had a dust bunny.  I would see it and be disappointed in how it partially ruined the image.  Before I could decide whether to clean it myself or have it professionally cleaned, the sensor removal system got rid of it.  I have owned Canon T2i (550D), T3i (600D) and T7i (800D) cameras...all have the automatic dust removal system.  I don't know other people's experiences with dust removal, but that has been my experience.