- When creating a brain wallet, should I choose the compressed or uncompressed version? Why?
- What happens if I choose the uncompressed version (which is what the official client uses, right?), and someone sends coins to the compressed address? Will I see the coins? Would I be able to spend them if I saw them sitting in the other address?
- Will I have to remember which option I chose, or the pass phrase is enough? Will other options keep popping up? (eg: super compressed)
Brainwallet.org doesn't make this terribly clear: choosing a compressed versus an uncompressed address is not choosing between two different versions of the same address, but rather two different addresses based on the same secret exponent. Sending bitcoin to one address will not be accessible by the other, as they are technically independent. With that in mind, we can address your second point first:
- Will I see the coins? Of course! You can see everything that happens on the block chain.
- Would I be able to spend them if I saw them sitting in the other address? No. Unless you have access to the private key for the other address, you cannot spend the coins.
As for the first point, it really depends on whether or not your client supports signing transactions with compressed private keys. Your client will always be able to receive coins at the public address, but it may not be able to process a compressed private key and thus not be able to spend with it.
Here's some info on popular clients that might be helpful:
BlockChain.info -- Currently, if you try to import a compressed private key into blockchain.info's web app, you will get a warning that they key will not be usable to create transactions on the mobile app. So, if you need to make payments with their mobile app, don't use compressed private keys. Although their web app, should work fine, even if accessed from mobile.
Bitcoin (Android - Andreas Schildbach)-- Andreas Schildbach Android bitcoin client, on the other hand uses compressed private keys. Although they are normally hidden from user, there is a (somewhat convoluted) way to import and export private keys into the app and out of the app. So, if you are making payments with this app, you can use compressed private keys. (Note: I'm actually unclear as to whether or not you can import uncompressed keys into this client, so compressed keys might actually be mandatory here.)
Armory -- I don't believe that the Armory supports importing of private keys at this time. So, if you're planning on using this, you must use uncompressed keys.
As for your third point, I would suggest remembering your chosen option just in case the clients you wish to use doesn't support it compressed keys. That way, you have a good idea of what's going on in case making a transaction fails.
It makes no difference which you choose.
For more info please see the answer to this question: What is a compressed Bitcoin key?
A compressed key is just a way of storing a public key in fewer bytes (33 instead of 65). There are no compatibility or security issues because they are precisely the same keys, just stored in a different way.