Comparison of Stainless Steel and Galvanised Steel in General Applications

Mechanism of protection A protective oxide layer which self-repairs in the presence of oxygen, granting long-term corrosion resistance. A protective zinc coating is applied to the steel during manufacturing. When damaged, surrounding zinc cathodically protects the exposed steel.  The stainless steel protective layer is more durable and is able to 'heal' itself. Stainless steel protection does not diminish with material loss or thickness reduction.
Appearance Wide range of finishes available from very bright electropolished to abrasive linished. Appealing high quality look and feel. Spangles possible. Surface not bright and gradually changes to a dull grey with age.  Aesthetic design choice. 
Surface feel Very smooth and can be slippery. Coarser feel which becomes more apparent with age. Aesthetic design choice.
Green credentials  May be re-used in new structures. Valuable as scrap after lifetime of structure - high recycling rate because of collection value. Carbon steel generally scrapped at end-of-life and is less valuable.  Stainless steel is extensively recycled both within manufacturing and at end-of-life. All new stainless steel contains a substantial proportion of recycled steel.
Heavy metal run off Negligible levels. Significant zinc run off especially early in life. Some European highways have changed to stainless steel railings to avoid zinc contamination of environment.
Lifetime Indefinite, provided surface is maintained. Slow general corrosion until zinc dissolves, red rust will appear as zinc/iron layer corrodes, and finally the substrate steel. Repair required before ~2% of surface has red spots. Clear life-cycle cost benefit for stainless steel if extended life intended. The economic break-even point can be as short as six years, depending on the environment and other factors. 
Fire resistance Excellent for austenitic stainless steels with reasonable strength and deflection during fires. Zinc melts and runs which may cause failures of adjacent stainless steel in chemical plant. Carbon steel substrate loses strength and suffers deflection. Stainless steel offers better fire resistance and avoids risk of molten zinc if galvanised is used. 
Welding on site Routine for austenitic stainless steels, with care about thermal expansion. Welds can be blended into surrounding metal surface. Post-weld clean up and passivation essential. Carbon steel readily self-weldable but zinc must be removed because of fumes. If galvanised and stainless steel are welded together, any zinc residue will embrittle the stainless steel. Zinc-rich paint less durable than galvanising. In sever marine environments, a crusty rust can appear in three to five years and steel attack at four years/mm afterwards.  Short-term durability similar but zinc-rich coating at joins required upkeep. In severe conditions, galvanised will get rough rust - even holes - and possible hand injury especially from unseen seaward side.
Contact with damp porous material (e.g. wooden wedges) in a salty environment Likely to cause rust stains and crevice attack, but not structural failure. Similar to storage stain leading to rapid zinc loss and longer term due to perforation. Not desirable for either, but in long term can cause failure at base of galvanised poles. 
Maintenance  Can suffer tea staining and micro-pitting if not adequately maintained. Can suffer general zinc loss and subsequent corrosion of the steel substrate if not adequately maintained. Rain in open areas or washing in sheltered areas is required for both.