The 125 has an extra square foot of surface area; but when it comes to decorating it, the 120 will beat it hands down. That extra 6 in. front to back provides greater sense of depth and also allow you to freely decorate tank without your dry area creeping frontwards. If buying used, try and find the tank date of manufacture sticker (ussually attached to bottom). Would stay away from anything older than 10 years or any tanks where silicon appears damaged, dry, brittle or harder than in a new one. Tanks can be resealed; but it's a time consuming job. You would have to completely remove all the old silicone before resealing and dealing with bad fumes. Good luck!