A 24 word seed is better than a 12 words. The highest the words the better the security.
Well, it's not quite as straightforward as that.
A 12 word seed phrase provides 128 bits of entropy. A 24 word seed phrase provides 256 bits of entropy. This much is true. However, all bitcoin private keys provide, at most, 128 bits of security. So the security of the private keys generated by a 24 word seed phrase are identical to the security of the private keys generated by a 12 word seed phrase (provided, of course, that the seed phrases were not generated insecurely).
12! It is safe for most GPUs to bruteforce, so whoever wants to try bruteforce should pay more to buy more advanced hardware ASCI and thus more time for you to transfer your money.
12! is very easy for most home hardware to bruteforce, as I explained above. Also, ASICs are useless when it comes to brute forcing seed phrases.
So, If you want to guess 12 words out of the 2048 and try to arrange them in the correct order gives you 5,271,537,971,301,488,476,000,309,317,528,177,868,800 possible combinations.
Not quite sure how you arrived at your number there, but it's incorrect.
Picking 12 words out of 2048 gives 2048
12 = 5.444*10
39But if you consider the checksum, then the number of valid combinations is 2
128 = 3.403*10
38
