Stewart,
They're both excellent products, and they both offer similar benefits when compared to using the VFP database engine. I'm sure other folk here will give you their views, but, for what they're worth, here are mine:
SQL Server has the advantage of being the market leader, with all the muscle of Microsoft behind it. It is much more mature than MySQL (been around for nearly 20 years), and has far more features.
I believe SQL Server will give you better performance than MySQL (but maybe not so much better as to tip the balance). It offers better security. And it has a better range of utilities and supporting applications (DTS, English Query, OLAP, Reporting Services, ....).
On the other hand, SQL Server can get very expensive, whereas that's not really a problem with MySQL. Then again, if you are selling a product to an outside market, the fact that MySQL is free might itself appear to a drawback in the eyes of some client companies.
Above all, remember that, with either product, you will have a lot of work to do in adapting your front-end application and migrating your data. But both products can bring a lot of benefits if you do it right.
Hope this helps. I'd be interested in hearing what you eventually decide.
Mike
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Mike Lewis (Edinburgh, Scotland)
My sites:
Visual FoxPro (www.ml-consult.demon.co.uk)
Crystal Reports (www.ml-crystal.com)