Yes there is a difference. Some of the older baitcasters and spinners for that matter used pewter for gears and that can corrode. Same for Magnesium, it corrodes too easily. You want all internal components made of either alumium, brass, or Stainless. And like Kurt said, they all still corrode if let's say 'dunked' for some reason and then not properly taken down. Also, never soak a reel in a bucket of water.. if one dunks a reel immediately take it to a shop or tear it down yourself taking pics before each item is removed and also laying out the parts in the order they came out the reel.. use both. take pics and layout in order.
I've been using Lews reels for the last 4 years and I'm super happy with how they've held up. Lews starting putting more attention to material quality and listening to what inshore anglers wanted.
The just released a new reel they call the Custom Inshore and it's a pretty sweet little reel. I'm fishing with their team Lews lite and I really like it. The Pro G is also super sweet.. any of those three and also the new BB1 Pro are all great and the specs on them make them saltwater ready.