The SSI tables are modified from the Navy Dive Tables to be more conservative. This was a result of testing done that indicated "micro-bubbles" in divers relying on straight Navy tables and the general fact that the average recreational diver is not in as good of shape as the military diver that the Navy tables were written for.
I have not seen the older PADI tables that you have, but I know that my IANTD tables within NDL's are actually more conservative than SSI's, but that is partly because IANTD allows planned decompression stops for overstaying. SSI does not.
Personally, I will plan dives with tables for "borderline" dives and dive my computers. I carry two Suunto computers of the same vintage. If one decides to give me a "no-go" underwater, I will dive the other one to a safe dive. On decompression diving, I will dive tables as well as plan by them to avoid going into Deco with inadequate gas for me and my buddy to do our safety stops.
I have not seen the older PADI tables that you have, but I know that my IANTD tables within NDL's are actually more conservative than SSI's, but that is partly because IANTD allows planned decompression stops for overstaying. SSI does not.
Personally, I will plan dives with tables for "borderline" dives and dive my computers. I carry two Suunto computers of the same vintage. If one decides to give me a "no-go" underwater, I will dive the other one to a safe dive. On decompression diving, I will dive tables as well as plan by them to avoid going into Deco with inadequate gas for me and my buddy to do our safety stops.