I'm voting for the 120g if you wanna go planted with it, the cube shape allows for some wicked aquascapes. Plus at the 4ft length, lighing is easy to find, and the extra front-back width, allows you to easily add more fixtures depending on plant needs.
I would stay away from the 5ft length tanks, they make lighting a headache trying to find the best solution. Even though the 6ft, can be easily lit with 3ft. fixtures, again I like the depth of the 120g.
And since this your first tank with a height of 25", good reflectors help out alot or removing the legs from your lighting helps with growing good foreground plants and new plants.
And if your lucky enough to be starting over completely with new lighting as well, I strongly suggest T5HO with individual reflectors, 6-8 bulbs should be enough, plus you'll love the spread and penetration (because of the reflectors) over PC bulbs.