I use this as my cordelette for canyoneering and trad climbs...EXCELLENT durability and strength...the only 2 things you give up with it is a little weight (it's lighter than a 6 or 7 mm of the same length of regular cord) and it's very low stretch (not dynamic) so don't expect it to absorb force if shock loaded. Once you figure that into your set up, then your ready. I have sat on mine dragging it across many sandstone canyons in Zion as i have squeezed through tight slot canyons and it BARELY retains a blemish. This is hardcore cord...no pun intended...
PS: Despite what Mr. Bohanon says below, it is not just a standard nylon cord, just read that product description above closely..."The 6mm PowerCord is stronger than standard nylon, breaking at a astronomical 4800lb." So it's as strong as a solid carabiner or any other part of the system (rope, harness, etc.) so unless your planning to hang your Mini Cooper from it, you have no worries. (but it still most likely would hold it)

Go with the tri-action...it's bomber solid...just keep it oiled for time to time (which may not be very often)
I own 5 of them and use them for everything from burly tasks like belaying to locking my 3 year old daughter's body harness on her in a way she can't ever undo...

PS. I FULLY agree with Matt's answer about the body harness...you can just add a chest harness to a regular seat harness, too. It will keep you upright at ALL times.
You can get that stuff here, but the rope may need to come from another place (i.e. Sterling)...(try googling "sterling shorts" if you want a cheaper shorter rope...)
Good luck, GO STATIC...

No rope is going to fail on you, but you must account for your weight, or if you slipped, and sharp edges on the rope, etc....
For rappelling ONLY, a static (non-dynamic rope is HIGHLY suggested)
I would suggest anything over >10 mm. And I would suggest a Sterling Rope if you want the best, but price may be a factor and Sterling isn't the cheapest (i.e. depends how much you plan to use it, but also remember you're hanging your life on it; used by rescue crews and awesome climbers around the world)

So for example, a Sterling HTP Polyester Static Rope (for rappelling ONLY; see sterlingrope.com for catalog), or for any chance of climbing a bit on belay or chance of a fall, I suggest Sterling Marathon Sport Rope - 10.4mm. I have this rope. (but remember that your 300 lbs will stretch a dynamic rope out like a rubber band and there is potential for the rope to go shooting out your rappel causing a catastrophe when you (or anyone) gets toward the lower portion of a rappel...

No climbing? STATIC ROPE ONLY PLEASE.

No, just the backpack straps. However, they are adjustable enough to be long enough to wear like a messenger bag. That's how i cart mine to and fro'.