May I ask, why not SX 814?
Slighly shallower well depth is the only main theoretical disadvantage (maybe a higher cost too). I used a camera with 674 chip before and did not notice any practical difference in well depth between 674 and 814. Sensitivity will be a tad lower due to smaller pixels, but images taken with 814 chip IMO look better than those taken with 674. Also, 814 when binned gives a decent 2.3 mp with 7.4 micron pixels and well depth of about 23000 (when binned 2x2).
Edit: 814 chip is also twice the size of 674. If money is an issue, then Atik 490 (814 chip) costs just as much as SX674.