Sometimes representatives from Power Companies will attempt to get you to change your electricity account to them because "They have better service." This is a ... bit of a stretch often.
Large companies hire large call centres. This means that you are likely to get through to someone quicker. (Mercury energy aside, their wait times are incredibly long). However it does mean you'll never reach the same person twice. Often in large call centres people do feel like 'bits of the machine'. People will often treat you like a moron and generally the centre is so vast the representative often doesn't know everything and gives you half truths.
Small companies however often have longer wait times. The reason for this is if they have 50 call centre staff, and a flu goes around and only 40 people turn up for work then you will wait 20% longer.
However small companies generally have more experienced people as the people know more about the business and how it works.
If you want to test the Power Companies service I would recommend calling their contact center and saying your electricity is off. How they treat you is probably an indication of their service.
Don't trust the sales person, always call the contact centre first. (There is a reason nobody at the Power Panda is with Mercury. Just saying).