Atari 800 vs Commodore 64
The Atari 800 was one of the best 6500 systems.
Better than a Commodore 64 in many ways.
The greatest advantage of the Atari 800 over the C64 was expandability.
The 800 had multiple card slots.
The later Atari XL/XE were worse by loss of expandability.
Atari was faster, its 6502 ran at 1.7Mhz, the C64's 6510 ran at 1.0Mhz.
Atari's graphics had more more colors (base colors plus luminance levels)
and more graphics modes (resolutions).
The graphics chip of the Atari (ANTIC), like the Amiga, was a coprocessor
that executed "display lists". Mixed modes were coded in the Atari
by the 6502 placing into RAM "display list instructions" for the ANTIC to execute.
Atari had a smooth-scrolling of its "playfield" which C64 had no counterpart.
C64's sprites were better, the only area in graphics the C64 had an advantage.
Atari's "player/missile" sprites had limitations in setting positions.
C64's sprites could be set in any position and were larger.
C64's sound was far superior with its SID chip.
For a user interested in music, the C64 would've been a better system.
In games that were ported to both, the C64's voices almost sound like real musical instruments,
while Atari's voices sounds like a primitive synthesizer.
Atari BASIC was better, having Atari-specific commands, and may have been faster.
The Atari had its own DOS and could boot into a useful notepad program.