Depends on severity and cleaning application:
General use/regular cleaning
I'd recommend backpack vacuums (e.g. ProTeam, Cen-Tec, Oreck Commercial, Perfect, Bissell Big Green, etc.) in 4-6 quart capacities. I'd discourage 10 quart capacity backpack vacuums for smaller people because overtime the filled bag can add 2-3 pounds. Backpack vacuums will clean fastest in square feet per hour. In addition, most backpack vacuum cleaners are easily convertible to Hepa level filtration with Hepa dome filters if you want to substantially reduce allergens and improve air quality. Moreover, most of them come with multiple tools to clean whatever surface(e.g. carpets, hard floors, crevices, corners, edges, high dusting, upholstery, blinds, etc) -- that way you don't need multiple vacuums. If you have a client that has pets, get an air-driven turbo tool to pick up the pet hairs. If you or your employees need to do high-dusting, most backpack vacuum cleaners can be equipped with extendable/telescoping wands so that a ladder isn't needed.
Plus in comparison to upright vacuums, they are by far cheaper in long-term maintenance/upkeep -- no fans, brushrolls/beater-bars, belts, & brushes to replace. They use regular extensions cords so in case you have rough employees who damage those, you don't have to shell out a huge chunk of money for a proprietary cord and the associated repair fee. Most of the major backpack vacuum wands use 1 1/2" diameter fitting attachments, so you can easily swap and replace them when they get worn-out or lost.
Most of the large national cleaning companies make backpack vacuums the standard. In fact the competition for those national cleaning companies is so intense that many of the big brand backpack vacuum cleaner companies sell direct to them bypassing distributors like me and sell to them up to 60% below market price.
The only major complaints to backpack vacuums based on feedback I've received form my customer base is that if you have a picky client who wants to see those "vacuum lines" on the carpet -- they don't show those with the default carpet tools, but you can fix that with a powerhead attachment.
I'd recommend dual motor upright vacuums with a Hepa filter & attachments (e.g. ProTeam, Oreck Commercial, Panasonic, Sandia, etc). They tend to be a bit heavier due to the extra motor, but you get way more agitation for plush carpets and dirt/debris are picked up much better. In addition, they have higher-quality parts so they do cost less in terms of long-term maintenance/up-keep. If you decide to purchase a dual-motor vacuum, make sure you get one with attachments because not all of them are equipped with those. With attachments, you can clean more areas besides the carpet.
Last edited by Plant Maintenance; 05-31-2014 at 10:52 PM.