Thinking I need one of these. 80's are much more plentiful, folks talk highly of their 70's.
What are the big differences?
If you're speaking of the actual Series 70, the difference is two-fold, i.e., 1) a prong style barrel bushing on the original 70, and the absence of a firing pin safety of any kind on the original (and current) 70. Basically, the 70 differed from the straight up
Government Model (i.e., the commercial name for the USGI standard sidearm till 1980) by adding a prong style barrel bushing. That became the standard commercial Government Model offered by Colt till the series 80, which replaced it. The 80 went back to the standard Government Model barrel bushing, but added an internal firing pin safety.
No one much cared about the dropping of the prong bushing, but many complained about the addition of the internal FP safety, and thus the 70 came to be highly prized by 1911 fans. Folks were opting for used Series 70 Government Models and Springfields over the new Series 80.
Eventually Colt got the idea (having lost many customers to Springfield and others who were making what amounted to old fashioned GMs, i.e., without internal safeties), and "brought back" the "Series 70," only it wasn't a Series 70. What they brought back was actually a
pre-70 Government Model falsely labelled "Series 70," just to take advantage of the enthusiasm for the old 70. Didn't matter, though, because few were particularly nostalgic for an original 70 for the prong barrel bushing anyway. They just didn't want an internal FP safety, so it made many happy.
It's all very complicated.