so your after a 3 foot tank, in that size i would go with an all in one as that will be much easier for you, buy it, place it & fill it.
If you go for a custom made one you will need to build a stand & hood, it will cost a similar price either way & the custom one will only look as good as the excellence that goes into making the stand & hood. A custom tank does give you the ability to have specific sizes, if you wanted an extra deep or tall tank then that would be easier. each method has their own advantages & disadvantages.