Nobody tries to "guess" a private key. Brute forcing private keys is for all intents and purposes infeasible. 256bit is a large number (likely a quadrillion, quadrillion times times larger than you "think" it is).
These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space.
http://www.schneier.com/blog/archives/2009/09/the_doghouse_cr.htmlUnless you are worried about attackers building computers from something other than matter and existing in something other than space the attack vector isn't to "guess" your private key/seed it is to
GAIN ACCESS to your private key/seed.Your coins will be stolen if the attacker GAINS ACCESS TO the private keys. For unencypted wallets this means access to the wallet file. For encyrpted wallets this means the wallet file and the passphrase. If the passphrase is weak the attacker may be able to brute force it. There is no likely scenario where an attacker would gain access to only some but not all of the random private keys but would gain access to the seed and thus all private keys.
Deterministic or random once the attacker has the decrypted wallet file, you should assume your funds will be lost. It is your job to ensure the attacker never gains access to the wallet (deterministic or random).
Now if you employ a second wallet (say offline "cold storage") it should use keys which are unrelated to the first wallet. This applies regardless of if you use a random or deterministic wallet.