If you add a new apollo then you would still be able to use the old one for inputs as well I believe? I may be wrong there.
My advice to you would be to consider how many more inputs you need. In my own research and refining the use of my Apollo MK II I've decided that I don't need all those inputs and will just be adding the satellite octo. To answer one of your questions that would be an additional 8 cores of processing power as well. Speaking in CAD $ I could get another OCTO satellite (8 more cores) for around 1200 or jump on the Apollo 8 deal and get the free satellite for a little over twice as much. My mixes tend to be pretty large and I'm using UAD plugins more and more so I'm also having the same debate. Even with track freezing as I don't freeze my busses.
Currently I have 6 corse of processing power and I'm always running out. I mostly mix / produce pop so most of the time I'm using one input at a time. Hey there! I currently run an Apollo twin MKII DUO and a Satellite Quad.