Of course there is a difference. When you sit the guitar is anchored and the drape of the right arm can be more flexible. The neck angle can be altered more easily by "pivoting" the body on your lap. The left hand can be braced against the body for bar chords etc. When you stand up, the guitar hangs as a pendulum on the strap and alterations of angle mean that the strap has to slide across your shoulders etc. You have all my sympathies because I find sitting down much easier too! But, some songs, some venues, some audiences and some line-ups do work better from a standing position. So, for what its worth - Study how you sit, where things are, what moves when you play. What needs to adjust when standing? Play standing up. What hurts? What doesn't work? What needs to change or be flexible? Some hints- 1. You cannot lengthen your arms but you can go for a guitar with a smaller/thinner body and a narrower neck. 2. You can think about your action and string guages. 3. Put the guitar in a comfortable postion and get someone to measure the strap length, then get someone to cut you a strap from a single piece of good quality leather. 4. If you want the guitar to stay still, have the "hide" against your clothing. If you need the neck to tilt have the shiney side against the cloth to reduce friction. 5. Practice before a mirror. If you detect an awkwardness have a look to see what is causing the problem. 6. Practice standing up. Develop those tiny muscles that are not used sitting down.

If you are still not happy, buy a high stool. Best of luck. Mark