Increasing the length is better according to The New Marine Aquarium by Michael Paletta. It allows more surface area for gas exchange, taller tanks require more mechanical assistance to keep the water well circulated, and wider tanks allow for more natural looking aqua scapes. These are some reasons for a wider tank given in this book.
As for where to buy your aquarium, I'm in the same position. I don't know whose good to use, so hopefully someone will respond and we can both find out
Good luck and take care!!