Oil based poly has an amber hue, whereas water based poly is clear. An oil based polyurethane will continue to amber and darken over time, while water based poly will remain clear for the lifetime of the hardwood floor. This color difference is less noticeable over a stained hardwood floor, but an oil based poly will still exhibit an amber hue that will continue to darken over time.
Oil based poly takes much longer to dry and cure, and only one coat of oil based poly can be applied to a hardwood floor in 24 hours. Water based poly dries much faster, and several coats (up to 4) can be applied in one day. Water based poly can be walked on with socks in about 4-6 hours after last coat has been applied, while oil based poly must dry for at least 24 hours before walking on it with socks.
Oil based poly has a very pungent smell and emits a strong chemical odor and noxious fumes while drying that should not be inhaled. Applying oil based poly requires the use of a respirator and your clothes will stink for hours afterwards! We normally advise our clients to be out of the house and also to remove their pets while oil based poly is being applied and drying. Water based poly, on the other hand, has almost no smell. Water based polyurethane is safe to apply without a respirator, and will not stink up the house like an oil based poly. It is safe for people and pets to occupy a house while undergoing coatings of water based poly.
Oil based poly is a thicker poly, but it is also softer. Water based poly, an acrylic product, is a thinner, but harder coating. Oil poly can be more susceptible to dents, while water based poly can be more susceptible to surface scratching. Because water based is a thinner product, we recommend 3 coats of water based poly as a minimum, whereas with oil 2 coats are sufficient as a minimum. For people with large pets, we recommend either 3 coats of an oil based poly or 3 coats of Bona Traffic High Durability water based poly.
Water based poly is a more expensive product. A 5 gallon bucket of oil base polyurethane costs us around $150.00 to purchase, whereas the same amount of water based poly costs us about $350.00. Expect to pay .50 - $1.00/sf more for a water based polyurethane finish. If your flooring contractor is not charging you more for a water based polyurethane, they are using a cheap brand that will not last! As previously stated, we use only Bona brand water based polyurethanes, as they are the most durable on the market.
I have heard some people say that water based poly is a rougher finish and doesn't feel as smooth as an oil based polyurethane. We apply both polyurethanes all the time, and if you hire an expert contractor, either finish should feel as smooth as satin under the feet!