Neither one of those decisions was a mistake, though, so this poll is poorly thought through... seriously, where is the "both of these were good decisions" option?
I know this has been said many times in the thread, but seriously, the title should not have the old, false "Nintendo betrayed Sony" lie in it, it's not true! Sony tried to take the SNES away from Nintendo because they inserted a 'we get all royalties from SNES CD game sales' clause, and Nintendo either missed it or didn't realize its importance until later. Once they did, of course making a console with Sony was out of the question, you can't let another company get all the licensing fees on your platform! So of course Nintendo did the right thing when they dropped Sony, it was the only move to make in that situation. They should have avoided it in the first place by pushing back against that clause in the first place of course, but oh well. As it is, this move was a good decision that helped Nintendo quite a bit and did no damage. In the end, not releasing a CD addon to the SNES was probably the right move; I like the Turbo CD and the Sega CD, but a SNES CD just wasn't necessary. Carts were fine.
And as for the N64 using cartridges, as a big fan of the N64, I have never regretted this in the slightest. Yes, certainly, the N64 was MUCH less successful in Japan than it would have been had it used CDs. Part of why Sony won Japan was because of people disliking how controlling Nintendo was being and would have happened anyway, but part was because of the N64 not using CDs, while developers wanted FMV cutscenes and such in their games. Square led the charge to Sony and helped convince other devs to follow, and they may have been less aggressive about that if the N64 also used CDs. The N64 might have sold better and been more popular had it used CDs. It would have had more games as well, most likely, whether or not it sold better, because of cheaper manufacturing costs.
However, the benefits of the Nintendo 64 using cartridges are, to me, much greater! Sure, a CD-based N64 would have had a chance of selling better and such, but it would have had worse games. Games like Mario 64, Ocarina of Time, Banjo, and the like, were almost certainly not possible on CD technology of the mid '90s. You run into RAM limits, and the scale of large areas in the N64 relied on the cartridge medium for very fast loading times, something impossible on a CD game. And then when going into the next area, it can load almost immediately on N64, instead of waiting for a long CD load! That games can load data quickly from the cart into the RAM is a core part of the N64's design, and it would be a significantly worse console with discs. This was a big part of why Nintendo went with carts back then, along with hardware-control reasons (only Nintendo can make the carts, etc.), and I do believe it. Some N64 games compress data on the cartridge and have loading times as a result, but most don't, and either way they have a significant advantage over disc based games in ways both visible and not. I would rather have an N64 with very fast load times and quick access to game data, than one with way more games and CDs, because those games would, in their gameplay portions, be on the whole not as good.
So, neither of these are good choices. One was a very good decision, and the other one has upsides and downsides and overall, for those N64 fans like me, is a definite positive. Even if a disccc-based N64 had led to Sony failing with their Playstation, I'd probably rather be in this reality where the N64 used carts because I do believe that for gameplay it was the better choice, and in games gameplay is what matters the most. Eventually, with faster disc drives and better use of clever programming to stream data from discs, disc games would catch up to cartridges, just about -- see the Gamecube, for example -- but technology was definitely not there yet in the early to mid '90s when the N64 was being designed. The N64 shows that in the mid '90s, cartridges were still better than discs.
