I've owned a Venture and I own a Concours. Let me spell out the differences to ya. In comparison to the Venture, the Concours will feel to you like sitting on a rocket. You will be leaned way forward and your feet will be way up high. The ergonomics between the two are so far apart as to be almost night and day. If you enjoy the Venture's ride, you need to be ready for a completely different experience.
There is literally NO COMPARISON between the two as far as passenger comfort - the Venture is a touring bike, the Concours simply isn't in the same league. However, the Concours can go into leans that the Venture would never handle with a passenger, so there's that. My wife preferred the Venture, but has said that the Concours is more fun for short trips out to the twisties.
The Concours bike will be lighter, lighter feeling, and way more easy to push around with your legs and at slow speeds. It will appear to you as a much smaller bike. The transmission and engine are completely different - the venture was comfortable at much lower rpms than the Concours is - you will constantly feel like you're over revving the Concours until you get used to it.
The mileage issue is anon issue - both of these bikes will run forever if you take care of basic maintenance. However, in my personal experience, the Venture comes essentially complete and ready to ride, and the Concours requires modification to make it really great. Almost everyone here subscribes to moding the bike with Murphs stuff, shoodaben stuff, and more. So add several hundred $ to the cost of the bike if you're gonna buy it.
Now having said all of this, let me tell you that I really like my Concours and I don't miss my Venture all that much, but I'm a multi-bike kind of guy and will be buying a larger tourer soon (hopefully) to add to the stable because my wife and I simply don't enjoy long distances on the Concours, and my other bikes are old, small displacement bikes.
