Orvis Battenkill

What’s the difference between these two fly reels?

Orvis Battenkill Bar Stock III $130 3.9 oz., 3″ diameter, for 5-7 line weights.

weights.http://www.orvis.com/store/product.aspx?pf_id=40C7&dir_id=758&group_id=768&cat_id=7627&subcat_id=7643

Orvis Battenkill Bar Stock Disc Drag $130 III 3.95 oz., 3″ diameter, for 4-7 line weights.

http://www.orvis.com/store/product.aspx?pf_id=44KY&dir_id=758&group_id=768&cat_id=7627&subcat_id=7643

What is the difference? Which ones better? What is disc drag? Also are both of these salt Water Proof?

I [...]