I use 4Change. I have switched several times whenever my current supplier gets too expensive or lazy. My last supplier (Proton) could never send an email invoice on time so I dropped them for 4Change. Their invoicing is timely. Whether to go fixed or variable: fixed prices will be higher usually for the supplier to cover the unknown future prices. I use variable pricing. The summer months might be high but the non-air conditioning months get real cheap. My winter invoices were as low as $50 for a 2400 SF house. Summer is higher but I have never went over $300. Before I retired I use to buy electricity for a Fortune 100 company so I am use to the variable pricing. Reliant will be the most expensive in the Houston area because they use to be the utility in Houston before deregulation. In Dallas they are more competitive because TXU is the old utility there.
There is no difference in reliability. All the electricity is the same over the same wires. The only differences are price and invoicing accuracy. If a supplier goes out of business (it has happened numerous times) you are moved to the price to beat pool until you sign up with another supplier.