The outside of the muffler and the seams will rust naturally, but take a look at the back of the muffler toward the bottom, there should be a small hole drilled in it. It is designed to weep moisture that leads to rust on the inside of the muffler. This may be your leak.
As for 14" vs 18"... depends on your taste. To get a good comparison you might try to find some sound clips on YouTube.com. Just input the muffler brand and the size, you should get some decent clips. A buddy of mine has the 18" Magnaflow, I have a 24", both single-out exhaust systems. The 18" seems plenty loud.
MODS LIST! -- 20% Tint, Curt Rec. Hitch, Slimline Toolbox, Huskyliner mats, Huskyliner Mud guards, 18" Gear Wheels, 33" Hankook ATM Tires, Bilstein 5100's at 1.4", Ported Foxbox w/2 10" Polk Subs & JL 500W Amp, Mufflex 24" Exhaust, Tow Hooks, Dominator II Step Bars, GatorHyde Spray-In Bedliner