I'm 6'1", and have a Marmot Helium in long. I haven't tried one in normal length, but I do not regret getting the extra length at all. I like to be able to snuggle up in my bag in the cold and doubt that would be possible in a regular length.
Depending on how you sleep, you might take up more or less room. I like to stretch out on my back and sleep on my side, so I probably gain a few inches in length stretched out.