125 gallons will most likely be at least 5 feet long... checked some "standard" charts and they list a 125 generally at 72 1/2 x 18 1/2 x 23 3/8. What does this mean to you? Easier time in the long run, since any change you make in the tank will have to be that much bigger in order for you to make a large mistake. Bigger tanks are more forgiving.
But... more water and larger tanks will most likely be more expensive. A large tank is expensive (unless found used), will require more salt mixture ($$), more pounds of substrate ($$), more pounds of rocks ($$, depending on if you pick up "live rocks" or just decorative), more lighting ($$, depending if you chose a fish only tank, reef, or
fowlr), more heating, and probably more money on fish (if you wish to add more fish compared to a smaller tank). BUT...
imo it's well worth it, especially if you are a fish nut like some people are here.
But don't worry, you could possibly sink the same amount of money in a smaller tank if you're not careful, so it's really up to your skills with
DIY, and your abilities to find good deals. In the end, it's up to you
~Rob