I would say that the basic difference is in who each software company sees as their customer.  Sage sees a professional book-keeper or accounting clerk, working on stacks of customer files, wearing those reading glasses that have a chain on them.  QB sees the business owner, or someone else with little or no accounting training, trying to crank out invoices and pay bills.  (This is my perception, and may or may not mesh with anyone else's)

Simply Accounting, maybe especially from the ACCPAC takover onward, was marketed to small business, and marketed through their accountants.  The focus was on building financial statements that were backed up by a solid audit trail.  Reports are considered as the gold-encrusted holy grail end product.  The ability to enter and recall general day-to-day workflow information varies from not bad (customer invoices outstanding), through awkward (partial item numbers) and onto nearly impossible (invoices relating to filled Orders).  If you are happy printing out everything on paper, and filing it in a well-organized and cross-referenced way, Sage 50 is your friend.

Quickbooks sort of grew out of Quicken, and tried to keep that Quicken level of ease of entering and organizing data first, then worked on turning that data into reports and statements second. 

As an example, in Quicken, when you create a 'job', it's linked to a customer.   When you buy some product for the purpose of completing a specific 'job', it's fairly straightforward to use the software to help you get all those purchases through to the customer invoice.  This is pretty much how it gets done in the real world, if you aren't ERP-sized.

On the other hand, the closest thing to a 'job' in Sage 50 - a 'Project' - can't be linked to a customer.  This makes it highly flexible, and just as highly prone to data entry error.  It's useful as a tracking tool for project progress, assuming every source document was entered correctly.  As far as using it for a billing tool... perhaps if you had an unlimited amount of time and paper and patience, it could be done.  You could fire up your printer and print out the project reports, then take your pen and a ruler, and cross out every purchase that has a matching sale for each Project.  Then maybe you would use a highlighter to mark the not-crossed-out items, so you could type them into an invoice.  If your last computer had those big tape reels, you'll feel right at home.

Tax software that integrates with Sage 50?  I think it's most common to use CaseWare to work up the file first.  If you have all the GIFI information up-to-date and correct in Sage 50, you can export the financial information by the GIFI codes.  The file format is .GFI.

I hope that helps,

Randy