I'm not sure your description of spritecount and game type rules any of the above machines out to be honest. I think it boils down to:
*The 7800 is an Atari (important for some folks), has a flexible display, is still 6502-based but there's a learning curve for the graphics hardware
*The coleco is z80-based (so a bit of learning to do, and not too obscure), the sprites would only be 8x8 though, so it depends how big you want them, or how many on a line. You could trade up to the sega mastersystem afterwards though - it's fairly similar to code for.
*The intellivision is nice, but the CPU is a little obscure and I found it a bit odd - but nothing that couldn't be overcome. You just wouldn't get so much reuse out of what you've learned compared to the other two though.
There's pros and cons for all of them, but nothing that screams 'forget about this machine' or 'it has to be this one'.
On a personal note I would suggest colecovision though so you can repurpose the code for the mastersystem afterwards - that's an underrated console I've got a lot of time for
Edited by sack-c0s, Mon Feb 27, 2012 7:04 AM.