I have both. At least for me gas is more powerful but more maintenance. Electric works great for everything but the most powerful washing jobs - like lots of oil and dirt on concrete. Be careful with too much pressure on siding and such as the water can gouge the surface. Hope this helps.
For what it's worth I had good success with this product:
Venus Laboratories Earth Friendly (Petastic) Pet Stain & Odor Remover
I was using it on carpet, not concrete though.
Saw it last night. Thought it was pretty well done. John Goodman was amazingly creepy and protectively friendly all at the same time. Thought they did a good job of wrapping up the questions.