Originally Posted by clm 
You are looking at the wrong lenses.
The expensive one is the SMC Pentax F 70-210/4-5.6 ED (extra low dispersion ). It has 13 elements in 9 groups and has SMC coating. I never had any of the others, so I don't know how much better the SMC version is.
The SMC Pentax F has a different optical formula than the Takumar F/Pentax F and has SMC. It does not have "ED" in the lens title although some people claim that it has an ED optical element. It also takes a 49mm filter. I'm sure the SMC F 70-210 is a better lens than the Takumar F, but I couldn't say how much better as I never directly compared them. But the Takumar F is a pretty good lens and it is fine for casual use.
Personally, I agree with clm who stated that the Pentax DA 50-200 as the logical upgrade for use with a digital body. It is smaller and lighter, has the Quick-Shift feature, is definitely an ED lens, is optimized for use with a digital sensor, and should perform at least as good as the older, larger and heavier SMC-F 70-210. I just looked at the O.P.'s profile and she has the DA 50-200. I don't know why you would bother with the Takumar F or consider buying an SMC-F 70-210. If you want top performance, then wait for the DA* 50-135 f2.8 or DA* 60-250 f4, which are likely to be outstanding lenses in the M*/A*/F*/FA* tradition. Another option would be a fixed focal length lens like the FA 135mm f2.8 or the soon to be released DA* 200mm f2.8. I have the FA 135 and it is a very high performance lens and quite compact for it's focal length.