I know you would be very pleased with what you can do with a 45 gallon as far as a planted tank goes. However with the 1.00 a gallon sale going on at Petco and Petsmart it is only on certain size tanks The tank sizes this applies to are 10, 20 High, 20 long, and 29 gallon for Petco any Aqueon tank over those sizes they carry in store would be 1/2 off the price. I'm pretty sure Petsmart is the same. Having a good amount of space in your office is a really great thin ( I know If I want space I have to go to the basement LOL) If you really have the space and since you are looking at the same length and width I would suggest a 75 gallon You may be surprised at how the size really does work. However it's not the space I worry about with larger tanks. It's the weight a 45 gallon tank weighs in the neighborhood of 376 pounds with just water in the tank this would not be with any substrate. With at the very least 2" of substrate in a 45 gallon you would need 90 pounds so that would increase the weight to 466 pounds, and then you would have other weight of decor so say 500 pounds total weight on a regular floor of a home You would need to make sure the floor could hold that weight. If you have an apartment you better ask the land lord for permission to have the tank and tell them what the weight is. Just in case the person living below you one day has a 500 pound aquarium in their apartment that they didn't want. If you have a cement floor under the tank you have no worries. (this is why I have all my large tanks in the basement I know the floor will hold the weight.
Just somethings to think about before you go to any expense. By putting a tank on a stand you also must know what that stand weight is combined with the total tank weight and make sure the stand will support the weight. I made a mistake like this years ago with a 55 gallon and it wasn't pretty when the stand collapsed under the weight and the apartment manager was not happy for the damage to our apartment and the one below us. It was an insurance claim on my part of over $15,000.00 that woke me up and also being thrown out of my apartment.
That's an awful way to learn about the troubles of a tank, large or small. I've always been paranoid about tank breakage/leaking, whether in an apartment, an office or house. And in apartments, we moved so much I never really got comfortable with an aquarium.
Had dreams for ages of coming in to find the tanks nearly empty and fish struggling to breath. I'm sure there's something psychological in that, but it does carry through to my waking tank planning hours.
Thankfully this is our house, at long last, and this is a good reminder I should make sure that the insurance would cover any major incidents. But there's no asking the landlord for permission anymore. (Although one of our previous landlords was the type to get excited over any pet additions we hoped to make, that was a nice change for many, many years there)
Last night I caught myself before I went all plant purchase crazy (which I do when it comes to plants of any kind, my yard is usually a master gardeners dream of unusual and unique plants), going between the different tanks. There is a HUGE difference in all of it when comes to 30 vs 45 vs 65 or higher tanks. Cost to get going, keep going, plans for the tank, where it goes, how heavy, what fish, etc.
Also have to make sure the stands can hold 400-650lbs. I have one stand I know will be fine but it is an awkward size.
Anything wider than 4 feet is probably going to have to sit in a window space of some sort, and while it's a north facing window in a very gray locale with trees that block most sun light, that doesn't mean it won't cause issues.
Going to consider it over the next few days as I finish organizing the office from our move back in after a major construction project, the 65+ gallon is tempting but I do worry I'm biting off more than I can chew so to speak. Not so much cost, because I'm well aware now that it will be an expense - I've seen what I can put into a 10 gallon in just plants! But more the maintenance, setup, and brooding over fish behaviors.
Then again, I can get a 30+ for one area and a 45-65+ for the other spot and see how it goes.. it doesn't help matters when one can get it delivered within 1-2 days of purchase.
Might have to purchase a lounge chair for fish watching though.