Every plumber and every plumber impersonator does water heaters, so looking at advertisements tells you exactly nothing about which plumber to call. You need references from your network of friends. If nobody here at MMM knows a good plumber in Houston, maybe Angie's List or Yelp can help.
Tankless water heaters are expensive to buy and expensive to install, since several things have to be changed to make them "up to Code." I, like others, have done some math and found the financial payoff to be uncertain.
I've installed a few hundred water heaters. Some are much easier than others, due to the unit's location. If yours is easy, almost anyone could do the job successfully and safely. Some companies, sad to say, will charge double if they can get away with it.