No, 51% is vastly more powerful than 49%. With 51%, you essentially control the entire Blockchain because you can always create a new Blockchain that would be accepted by the network, given enough time. Always.
With 49%, you can only get away with it a few times, and it's less likely you will mine the next 6 blocks.
Essentially, as time progresses, with 49% you lose out, with 51%, you keep winning.
I think I understand now. That would still be a bit of a tricky position because it isn't so much 51% when you commit the fraud that is important it is the period following the fraud.
How detectable would such an action be? Wouldn't other systems be able look at the block and say "it's verified, but it don't look right to me"
Not really. There are always lots of 'versions' of the blockchain floating around. The network only keeps track of the longest chain (broadly speaking). This means the network with 51% can determine which transactions get into the blockchain. For example, if the pool owner doesn't like you, he can essentially 'blacklist' your account, which means your Bitcoins can become unspendable, basically. The longer they have the 51% power, the more damage they can do. GHash already performed what you describe as your '49% attack' against a gambling site that accepted 0 confirmation deposits. There has been no known instance of a 51% attack yet (e.g. double spend after 6 confirmations).
You pretty. Much have to detect the fraud before/as it occurs - the dominant miner being silent for a while is a good indication they are building an alternate chain in private, for example. At least, that would be an indicator for a double-spend.
An honest miner with 51% of the hash rate would only mine 51% of the blocks (example: https://blockchain.info/blocks ); an attacker would mine every single block.
Essentially, as time progresses, with 49% you lose out, with 51%, you keep winning.