I would save the 29g to use as the sump for your 90g. That's what I did! 
If you want a 90g SW tank, wait until you can set up a 90g tank. I have a feeling you're going to go through the hassle of setting up the 29 and then quickly decide to work on the 90. Like everything else in this hobby, stay patient. Even if you're fiending to set something up, I'd wait and do it exactly how I want it done, however long it takes.
Plus, running a 90g SW will be a lot easier in the long run than a 29g - too small and too prone to wild changes in water quality real quick.
If you want a 90g SW tank, wait until you can set up a 90g tank. I have a feeling you're going to go through the hassle of setting up the 29 and then quickly decide to work on the 90. Like everything else in this hobby, stay patient. Even if you're fiending to set something up, I'd wait and do it exactly how I want it done, however long it takes.
Plus, running a 90g SW will be a lot easier in the long run than a 29g - too small and too prone to wild changes in water quality real quick.
