ITT: people think mining 6 1-minute blocks in a row is as difficult as mining 6 10-hours blocks in a row
Unless I missed something, then that's correct, but only if you interpret it in a certain way.
With the same relative hashpower vs. the rest of the network, over the same # of blocks, chances of finding X blocks in a row are the same, doesn't matter if avg. time/block is 10 seconds or 10 days.
But... wouldn't a theoretical attacker care more about how much time it takes to get a successful double-spend instead of how many blocks?
Yup. Faster blocks mean more attempts can be made, and thus the less secure the chain is.
Either way you would have to put together 51% of the hashpower of the network to pull it off, I guess longer blocks could make the attack more inconvenient, but they would probably still do it if they had the capability.
51% just gives you a high probability of succeeding. 50% can succeed. As can 49%, 40%, 20%, etc., they just have lower chances of succeeding. If someone is intent on double-spending, they will attempt it more than once. Again, the faster the chain, the less secure.