Here's my take on the level of quality on the rear bushings:
1. FreeMotion (bearings) -or- Elephant Bronze (it's hard to pick).
2. The harder 70D poly bushing.
3. I don't even recommend the softer poly bushings you're looking at for all the reasons that Rich mentions.
The more expensive ones (Elephant/FreeMotion) will hopefully last a life time and as mentioned, this isn't a job you want to do every couple of years. I'm using FreeMotion on my 914 and Elephants on my 911.
I used the soft poly bushings on my 911 years back and they distorted and squeaked. As mentioned, I replaced them soon after I got them (well... a couple years but, that's too soon to be doing this job again). I've even seen them take out front suspension pickup points on a 911.
The harder Poly bushings do not exhibit the same wear characteristics of the softer poly (meaning, they'll last a long time... if properly taken care of, probably as long as the stock ones did), they won't squeak and give virtually the same characteristics as the solid bushings.
With the harder poly bushings you will need to prep them properly to get them to operate noise free. They need to be honed to a perfect fit on your arm otherwise they can bind and clack or, if they're too loose they can do the same. Here's what I like to use now:
http://www.mcmaster.comItem #4719A261 Sleeve
Item #4650A68 Drum
With this is a lower speed drill press you can virtually guarantee a perfect fit on your control arm shaft. It only takes a few minutes with this set up and it's very precise.
Once the bushings slide freely on the shaft (while cool) you can install them and put zerk fittings on the arm to keep them lubed.
At $20.00 a set, I find them an excellent bushing if your budget is such.