Electric vs. Gas Powered, I'll make this very simple. They make electric pressure washers, but having access to electricity is most of the time not the case. Electric pressure washers are OK for some things, but are not very good in the commercial pressure washing business. Go for gas powered!
Hot Water vs Cold Water, If your budget can swing it, consider a hot water pressure washing unit, but if it cannot, cold water machines can work just as well. There's plenty of jobs that simply do not require hot water. There's many thriving businesses running just cold water units, but hot water does have it's advantages. Pressure washing isn't much different than any other washing - hot water is generally better, but cold water will work. Cold water will do OK for most residential jobs, but if you are considering commercial surfaces, construction equipment or anything else particularly greasy you will need hot water pressure washer.
PSI = Pounds per Square Inch. The higher the PSI the harder is shoots the water. 3,000 - 4,000 PSI is a respectable and plenty useful PSI.
GPM = Gallons Per Minute. How much water can the pump puts out per minute. The more water you can dispense per minute, the more ground (or surface) you can cover in that same minute. I like at least 4- 5 GPM for better rinsing capabilities. Commercial pressure washers are usually 5 GPM or more.