Same 64-bit preimage resistance security for SipHash and SHA-512/64? If I have to chose a 64 bit preimage resistant hash function; will there be any difference in security between 


SipHash and
SHA-512/64 (SHA-512 truncated to 64 bits)?


How long will it take an attacker to find a 64 bit preimage (e.g., minutes, weeks or years)?
SipHash doesn't claim to be a secure hash function. Only a secure MAC. So if you try to use it as a hash function, with a constant, public key, you are on your own.

SHA-512/64 should be a "secure" 64-bit hash, which is of course not enough for a truly secure hash, since it only has 32-bit collision resistance. However, since you only desire preimage resistance, it may have its uses.


  How long will it take an attacker to find a 64 bit preimage (e.g., minutes, weeks or years)?


That depends on the attacker's computational power, of course. With a typical desktop CPU you can calculate around $2^{20}$ to $2^{25}$ SHA-512 hashes per second. So a brute force preimage search would take maybe $2^{40}$ seconds, or tens of thousands of years. GPUs could reduce that by a large factor, but it would still likely be centuries.

However, the whole bitcoin userbase is currently (as of early 2016) hashing over $2^{60}$ hashes per second, and even taking into account a small additional factor from SHA-512 vs. SHA-256, you'd be inverting the hash in less than a minute at that rate.

If you want something that takes longer to invert, you should use a password-hashing function, like scrypt with sufficiently large parameters.