The people set up to provide on-site, customized support are the independent consultants. If your system needs an overhaul, a small independent consultant will work fine. If you need a new, customized machine, where you want to know the exact components that go into it, the small stores work reasonably well, and most independents will also build machines to order. For a new machine with no configuration necessary, and if you don't plan to upgrade much, chain stores can work, as long as you get everything you will need at one time. This includes software, hardware and service contracts.
Cost can also be a consideration.
Needless to say, on-site is generally the most expensive, but it also covers everything that might be wrong, and all of the hardware that could be causing the problem is right there next to the machine. Usually there is at least a one-hour base charge for the visit, even if it only takes 10 minutes to fix the problem. Telephone support is usually also available for about half the 'visit' cost.