I would suggest you try to feel the reel in hand on the rod that you intend to use it with first Chris. The comfort factor is a huge thing when fishing for GTs.
A Stella 10000 with 16000 spool will give you about 250 metres of PE8 line capacity against about 300 metres on the 18000 and that is, as Brandon indicates, a significant difference. An 18000 will also be cheaper than a 10000 with 16000 after market spool. On the other hand, an 18000 is quite a bit heavier.
Also, although I have never heard of a 10000 just failing with a big fish on, we should all remember that they were not really designed to be fished with over-sized spools (Shimano don't make one, which has to mean something) which allow the targeting of monster-sized fish. I fish with a couple of 10000s quite often and I must admit this fact gets into my head sometimes.
As is usually the case in our brand of extreme fishing, there are lots of pros and cons to assess there!