I know I posted a rubbish flamebait comment earlier, and possibly here comes another, but hey - it's just a slightly odd comparison anyway, really.
The ZX Spectrum and Commodore 64 were both very competitively priced and blew the real competitors in their own marketplace away. They were the most successful in their respective price ranges. The Spectrum left the likes of the Oric and Jupiter Ace for dust, because at the lower end of the market it seemed the best thing on offer. The C64 left earlier competitors such as the Dragon and the TI99/4a trailing in its wake
Looking upon the computers as mere games machines - as I suppose we are liable to do on a forum dedicated to gaming - the margin can seem more slender. But the C64, with its array of expansion ports, was a far more complete computer than the Spectrum - and rightly so, you paid for that capability. The C64 itself comes second best in comparison to the BBC Master, but of course that was in a higher price bracket. The fact that you *can* attach joysticks and other attachments to the Speccy to make it more expandable is missing the point - you're then talking about laying out a lot more cash, nullifying its price advantage.
People who admire the lovely detailed graphics of the Spectrum are forgetting that they look better on modern screens and under emulation than they ever did on our old tellies back in the 80s. Game developers could easily have adopted hi-res mode on the C64 if they had thought that having the detail was more important than having the colour. But it often made more sense to double up the pixels horizontally anyway, because if you didn't you'd end up with chroma noise distortion. On some machines, like the Atari XL, this was used as a rather lateral method to add colour to an otherwise monochrome (and I *mean* monochrome - the Atari's hires mode genuinely *is* that) screen.
It makes no sense going on about the Speccy's CPU "power". The Z80 was a knock-off of the Intel 8080, a processor that was outdated by the late 70s. The fact that it had a higher clock speed was irrelevant 90% of the time - many comparable instructions took more processor cycles to execute anyway. Still, the computational advantage might have held sway on some wireframe games, but lets not forget that those sorts of games were crap on all 8-bit machines, apart from the odd game that transcended the machines' limitations such as Elite, which made the slowness tolerable on all the formats, C64 included. Battle Command was a great game, and why? Because the C64 had a cartridge port as standard, which meant you could dump a load of look-up tables on the cart and hey presto! The slow computational power of the CPU was countered in one fell swoop.
C64 owners weren't short-changed by the CPU - the investment was made in the custom chips, which take a huge load off the CPU, while the Spectrum's CPU is expected to handle just about everything. The C64 could've been given a Motorola 6809 like the Dragon, as this was considered much more powerful than either the MOS 6502 or the Z80 - but it was much more expensive (I think the 6809 cost something like $175, when apparently MOS Technologies could manufacture 6502s for about $30). The budget for the Dragon was spent there, and we all know what Dragon games look and sound like as a result. Those who give short shrift to the C64's sprite capabilities should look at their nearest rivals - the Atari 8-bit, on which the sprites are few and splodgy, and the MSX, on which the sprites are many but tiny.
As far as games were concerned, the Spectrum does get a few victories but overall, it's men against boys, for me. Comparisons like Chase HQ are often a source of amusement to me. Okay, so it's an utter disaster on the C64. The Spectrum version is praised widely, but I think a lot of that admiration stems from getting more out of the machine than anyone thought it capable of. But it is better than the diabolical C64 version, so Speccy fans rejoice at their format getting one over the big beige one, without realising that it's still not actually much cop. I remember expecting quite a lot out of Speccy Chase HQ as a result, but actually it's pretty sluggish, and playing it is a pretty miserable experience next to the C64's big hitters such as Powerdrift, Turbo Outrun and Turbocharge.
TL;DR: refer to my previous post.
Soon you will have forgotten all things: soon all things will have forgotten you. (Marcus Aurelius, Meditations, Book 7)