One of the questions I hear with some frequency is "what's the difference between a CIO and a CTO?"   Having been both, I think I have some insights that might be helpful.

First let me talk about what I think they have in common:

• In both jobs, a key role is helping technologists understand what the business needs and helping the business understand what the technology can do for them.
• Both roles require a strong technologist with a strong grasp of business (kind of a corollary to the last point, but slightly different). 
• Both should be strategic thinkers.
• Both should be excellent leaders.

Now for their differences:

• I see a CTO as primarily focused on the top line while the CIO is primarily focused on the bottom line.  There's some cross over, but I think this is a valid distinction.
• A CTO is primarily concerned with external products and customers while a CIO is primarily concerned with running the business (internal products and customers).
• In an ideal world, the CTO runs the product development organization while the CIO runs the IT organization.
• If you have to choose, being a strong technologist is more important for the CTO, while being a good manager is more important for the CIO.
• A CIO has to be operational and understand how to build repeatable processes, reliable systems, and the organization to run them.  A CTO doesn't necessarily have to have these skill if backed up by a strong operational person in the role of CIO. 

A large technology oriented company (more than a few hundred employees) should have both.  There's too much to do for one person and the thinking can be very different.    One of the big problems at Excite\\@Home was they never had someone  at the "C" level who was looking internally.  "IT" was a division (not even a VP slot) inside the larger technology organization.  There were four levesl between this director and the CEO.  The result was real chaos in the internal systems and operations areas.  The CTO was a brilliant technologist, but not very "operational" and consequently, repeatable processes were hard to find. 

Personally I've enjoyed both roles, but I found the challenges to be very different.