I would say it depends on the language you are coming from, but if that's not an issue I would start with Swift . It is way more modern and overall more fun to use and faster to develop with.
The main advantages of Swift IMO are:
It is designed to be safer. e.g. types are not nullable unless you specify differently
It is a bit more geared toward functional programming, functions are first-class types
It is probably easier to maintain since you don't have two files per class, plus the code is cleaner
There's a playground where you can experiment with code which makes it easier to learn
On the flip side, depends on the scope of your project but you might not be able to use only Swift. There are a few things Swift can't do (yet), and there are a lot of third party libraries that are not for Swift (again, yet). The tooling is not fully there as well, but it is actively being developed and improved.
Oh and the most important thing - you can write code with emojis!
Awesome. I also had this query. Surprised, @joe hasn't jumped into the discussion yet! :P And yes, emojis are great. :)
