If Xur is selling an exotic that you own and also offering an upgrade for it, you are better off buying and not upgrading, right?
If you upgrade it costs you 14 strange coins (7 coins to turn in plus 7 coins for Exotic Shard), plus 7k Glimmer.
If you buy, it is 13 strange coins and it's yours.
So if he isn't selling what you want but is offering an upgrade I will go for it, but if the choice to buy is there that is the way to go, right?
Edit: Exotic shard may not factor into the scenario of owning both, since you could dismantle the one you own and get an Exotic Shard. It would still cost 14 coins and 7k glimmer I believe. Also, will upgrade cost be consistent for both weapons and armor?