I'd get the biggest tank possible because with this hobby, it seems like you only add to the tank.. corals, fish, inverts, etc. There is always something fun to add so I'd get the biggest tank your space/budget allows.
Me personally, I'd also like to try a glass tank vs. an acrylic. I really like the rounded edges of acrylic, but I don't like how easily it scratches. Acrylic is easier to cut to make a closed loop though, which does make it appealing for a lot of people.
I'd want to try a closed loop if I ever did it over. Powerheads are not so attractive in the tank and they always seem to break. If you are planning a tank with corals, I'd do a closed loop.