Dell or HP for Small Business?

fruitbane asks: "I work for a medium-sized non-profit, approximately 50 full-time users and 100 desktop PCs. We're redoing all our technology plans and budgeting; that means it's time to pick vendors/brands and stick to them, something we haven't reliably done in the past. Sites like Consumer Reports review various PCs and manufacturers for home users. Are there any comprehensive reviews or advice sources for those trying to determine the best vendor/manufacturer for small business desktops and laptops?"