The 4.50 is a 6 ply, that is why it weighs more. Most riders here either go with the 5.10-18 (6-ply) or the 4.60 (4 ply). I haven't heard of anyone using the 4.50, probably because it was not available two years ago.
Being an oddball, I chose the 120/80-18 4 ply because it was the lightest of all. I haven't changed my mind in the ensuing 4 years. I got a chance to swap bikes with a friend with the 5.10 on some moderately loose gravel roads and found the 5.10 to be slightly more stable when leaned over, and less likely to turn in as quickly. For slow rocky technical stuff or sand I would prefer the 120/80 for the same reasons.
On a light bike like this I don't see any reason to go for a six ply, but then I ride rather slowly and carefully, avoiding sharp rocks most of the time. If you really like to bash through rough stuff, get the 5.10.