Correct, PADI AOW and SSI AOWD are not the same thing. PADI AOW is equivalent to SSI Advanced Adventurer. There is a little course work and then a sampler of one dive from each of 5 specialties to earn the AOW/AA card. That one dive can then be counted towards the full specialty. SSI AOWD is an automatic recognition, not a class, that is automatically awarded to you after you complete 4 specialty courses of your choice and 24 total logged dives. In a way, SSI AOWD is a higher because it requires you to have taken four full courses, plus logged a few more dives, but they can be any four courses. PADI AOW requires that you do one dive each from deep and navigation (I think).
As for your other question, there is no problem bouncing back and forth between the two at the recreational level. Once you get into the professional pathway it becomes a little trickier. I did OWD with PADI and everything since with SSI.