I, personally, prefer a hybrid solution. First, guessing of keys will always be possible for a user who can ascertain their length. The possibility of them guessing correctly is low, but as you begin to use up possible keys it will inrease. In solutions I use, I tend to have honeypot sessions, unused, that if accessed ban a user temporarily. That takes care of random guessers who try to bruteforce.

I also log at a basic level users attempts to login (by ip). It's my responsibility as an administrator to decide if a user is attempting to hack the site, or is having genuine problems.

Furthermore, I _do_ use two session IDs. One of those session IDs is only used securely, though, by setting the cookie to encrypted. That session ID is used for higher level authentication procedures.

Even moreso, for anything that involves money, or purchases I require users to input their passwords if their session wasn't created recently.

That keeps sessions pretty damn secure.