If you can reasonably afford the exact tank you want, and you’re serious about remaining in the hobby, go for the custom.
I get the feeling you’re like me. I’ve wanted a reef tank since I was 5 (I’m turning 30 this year). I have a 120g tank at the moment but things have opened up for me to get a bigger tank. So I’m going for my dream tank size (8ft 240g). I’m going for it. It’s probably going to mean getting a new tank rather than used, which will take me a decent chunk of time to fund. But I’m trying to get one tank set up and plan on it being the one tank I’ll have for the next decade or more. Am I impatient to have beautiful coral and fish? Sure. But I’m willing to wait and take the time to set up my build exactly as I want it. So I’m taking my time.
all that aside, that’s my approach. Probably not exactly the same as yours. But my point is, if it’s what you want, and you’re serious and realistic about the costs (and can make it work without bankrupting you), go for it! In my case I would be upgrading to the bigger tank eventually. Might as well just go for it at the outset rather than paying for one tank and then taking it down and upgrading in a year or so.
so up to you, but that’s my approach