I climb for fun, and you'd be surprised at just how strong some of the accessory cords can be. 5.5mm Spectra(4000 lb tensile strength)and 7mm Perlon are the 2 main sizes I use for threading hexes and such with cord. I'm not familiar with the cords stocked by REI(mainly because I order in bulk, direct from the rope companies), but to give you examples I'll use New England Ropes
http://www.neropes.com
One thing that you need to be cautioned about is that accessory cord is not designed to be used as a climbing support. It may be strong enough, but there are differences in manufacturing and caution rules when it comes to equipment manufacturers. Here are the listed strengths for the NE Ropes Accessory cord. 3mm 405lbs, 4mm 720 lbs, 5mm 1125lbs, 6mm 1620 lbs, 7mm 1200lbs, 8mm 2875 lbs.
Technora cord (Tech Cord) from NE Ropes comes in 3mm and 5mm diameters. the 3mm is very light, at about 7/10ths of a lb per 100 feet, and the 5mm is about 1.7 lbs per 100 ft( I don't know offhand the weight of 550 cord). Tensile strength on these 2 cords are 3200 and 5000 lbs, respectively.
Tech Cord is used as emergency rappel line by some people(though you really have to take care about abrasion, if it's not going to be one time use), and if used as such, make sure to get a micro cord sized descender and learn how to use it, as part of the friction with a standard descender depends on the diameter of the cord.
The main difference between a Tech Cord and an Accessory Cord is that a Tech cord is designed to handle a much higher shock load, whereas accessory cord deals mostly with static loads.
If you're looking for a high strength low weight cord, you can't go wrong with the 3mm Tech Cord from NE Ropes(and yes, it even comes in black). The Main negative about tech cord vs 550 cord or accessory cord is, because it is designed to handle a much higher load as compared to it's diameter, is that it is very exensive(IE $700ish for 1000ft). Accessory cord is still more expensive than 550 cord, but much less expensive than tech cord. (prices, 3mm $0.12 a foot, 4mm $0.17 a foot, 5mm $0.24 a foot, 6mm $0.30 a foot, 7mm $0.37 a foot, 8mm $0.45 a foot.)
Oh yeah, one last comment, for those of us who do climb bigger walls, and need to haul a pack, we generally prefer a 8-9mm cord that is designed as a haul line. The reason we use cords like this are abrasion resistant(you have no idea just how much abrasion you get on a wall until you've been there) and they can be used as rapell lines in a pinch with the standard descenders that we use for our normal ropes(for me, on walls, it'd be either 11 or 11.5mm).
If you've got more questions, just lemme know.
TC
