Sell the tan. Agree with Kat. So many bags offer the tan colour and do a good job of it. The ivory Paddy is gorgeous and so now! Perhaps in a yr's time or two you may want to get rid of the ivory but for now it must be a keeper!!!
I just don't know. Tan is a neutral color that you can carry year round, while white is more spring/summer. Although some are now thinking the winter white/cream color can be worn all seasons. But if you have had the tan for a while, maybe you are looking for something new. And isn't the leather different on the old vs. new Paddies? Then again, you can get a lot of bags in tan...so it really depends on how you use the bag and whether you need the all-season neutrality. Probably not helpful...sorry.