I can't tell you how many times I changed my mind about some piece of equipment (although I did if you read my thread all the way through

). I went from a 33 gallon long with internal everything, to a 40 breeder with internal, and then decided to utilize my 65 gallon for my sump and keep it in the basement, out of the way of my display tank upstairs in my living room.
Many people wondered why I wouldn't use the 65 gallon for the display and my 40 breeder for the sump. First of all they have the same footprint, meaning you can't keep any more fish in one than the other (same surface area). Secondly, I hate working in tall tanks. And lastly, my 65 gallon allowed me to have a large refugium with all kinds of failsafes against flooding due to it's size and because it's larger than the 40 I filled it all the way up and used the return pump to fill my 40 upstairs.
If you take your time, and really dive into every little thing and explore all the possibilities, when you do put it all together you'll be rewarded with every well thought out decision. give us a link to your build so we can follow along and offer advice when you need it.