I would like to understand the pro's and con's of choosing a boat where I am in the higher end of the paddler weight specifications, versus the lower end. I am relatively new to whitewater (2 years), and am a little timid. I am 5'4 and 120lbs. I am currently paddling a Mamba 8.0 which I know is too big for me, but which I find incredibly stable in the class 2 and 3 water I am currently paddling. I am considering buying a Mamba 7.5, where I would be close to the low end of the suggested paddler weight range (120-170), or the Diesel 60, where I would be near the top of the recommended range (60-130lbs). Thanks for your input.

If you are planning on river running (your kayak choise is river running seemingly) you wold want to be in the lower spectrum of the weight range. The closer you are to the top of the weight range the lower you will sit your kayak in the water. This means that all the edges/chines will be close or in the waterline of your kayak. This means that the current will have a bigger influence on your kayak as the surface area making contact with the water is much larger, pushing you around and catching edges etc. You will however find the kayak tobe more responsive for turning etc in certain situations and you will be able to melt big holes better. The lower you are in the weight range the higher your kayaks edges/chines will be away from the waterline, this means that the current will have less surface area to grab and push you around. So it will be more stable as you are floating higher, it will be slightly more difficult to melt big holes but will boof a little bit better in certain situations.
So for river running/creeking you would rather be at the bottom end of the weight spectrum, not only for the reasons mentioned above but also you must remember that the gear you take along when paddling also has weight. So being at the bottom of the weight range means that you can load up gear and will still be good to go, if you are at the top end of the weight range and you load gear for overnight trip or something you might find yourself sitting very low in the water and battling to control the kayak.

If you were looking at a playboat the same as above would apply. If you are at the top of the weight range for a kayak it will be easier to get the ends of the kayak into the water for certain things but you will be less retentive in a hole feature because you will have a lot of your kayak making contact with the green water/outflow. Being at the bottom of the weight range means that you will be more retentive in certain features but you will not be able to get your ends into the water as easily. BUT with good technique you will get it all going well.

ps. other good river running kayak that will allow you to play a little bit as well is the Fluid Spice and for your size a medium should be good. If you ever make it to South Africa let me know then you can come join us on some of our local rivers.
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Thank you, that is very helpful. There is a possibility, although remote for this year, that I will visit South Africa! If so, I would love to take you up on the offer. Same goes if you visit Pennsylvania.