QSI cameras have been getting
rave reviews. It seems that, being a newcomer on the market, they have benefitted from the experience of previous cameras in their design. It looks like you use the QHY5 for autoguiding and since that works with the QSI 540wsg, there won't be any extra expense involved. It seems that the advantage of the qsi is the fact that the pick off prism is in front of the filters, so you don't get problems finding a guidestar when narrowband imaging as is sometimes reported with the sbig (and I'm no expert here--just quoting what I've read)
Geoff
Note added: Whatever camera you buy around the $US5000 mark, I suspect user skill will be far more important as a factor than camera performance.