The long and short is that in general a higher wattage power supply will have a higher amperage rating on the 12v rail (assuming that it only has one 12v rail.) I may be slightly misinterpreting your question, but both of these power supplies have one single 12v rail.
As far as I know, a single 12v rail is widely regarded as "better" or more reliable than multiple 12v rails under load.
The major difference between these PSUs, other than the rated wattage, is the efficiency rating. The 1300 G2 is 80 Plus Gold rated, and the 1000 P2 is 80 Plus Platinum rated. This means that the 1000 P2 is actually slightly more power efficient than the 1300 watt, at certain load levels (typically tested at 20%, 50%, and 100%). Both of these power supplies would probably be considered somewhat overkill for the average gaming build these days, but it all depends on what is in your system. If you are looking into 3-way or 4-way SLI or Crossfire, then you will want to take into account the power draw of the cards you are using, in addition to the rest of your system. There are numerous PSU calculators you can find online, although I would say you should trust these as a general guide at best. Either way, both of these PSUs are solid options, and are built to last.
For reference, I own a SuperNova 1000 P2. It's a great power supply, and probably a bit overkill for my system. My roommate owns the 1300 G2.
If you list the specs of your system, or system you plan to build, we can help you make the right choice more easily.