Suction side is from your skimmer/main drain to the pump. Pressure side is from the pump through the filter, heater, valves, etc out the returns.

One thing to be aware of is that the 3/4 HP pump option will require some additional plumbing so the spa can share the filter and heater while still maintaining separate pumps, assuming you don't have two suction and return lines for the spa. You will need additional valves and lockout protection so that the pumps cannot run dry. There might be other creative ways to plumb both pumps to minimize the rework. Do you plan on doing this yourself?

By the time you add the extra valves required for option 2, you may find that option 1 is actually cheaper and much less work. Also, you might be able to sell your old pump on Ebay or Craigslist :-).