The 8 an X are roughly the same size, but the X has a taller, crisper display. They are effectively the same width.
The 8+ and X both have a dual camera system, but both of the X's cameras are optically stabilized, whereas only the wide-angle camera on the 8+ is stabilized. They also both have 3GB RAM, but the 8+ is physically wider and supports landscape orientations and layouts in many apps.
If you're the type to keep your phone for many years, then I'd recommend the X. Yes, it's more money, but it's a really solid phone that has most of the advantages of both the 8 and 8+ and few of the downsides.
Alternatives:
1: There's supposed to be an SE2 in the works, which will be the same size as the iPhone 5 that you're used to, but with upgraded guts. I'd expect an a11 SOC and 2GB RAM.
2: If you don't keep your phone for years and years, then I'd recommend the 7 or 7+. The A11 is nice, and is the fastest SOC Apple makes, but the A10 isn't exactly a slouch, and the 8 isn't a HUGE step over the 7.