Stansted is the Stansted Express from Liverpool Street. The station is a bit like Gatwick where it is right at the terminal- a 50 minute journey time-
https://www.stanstedexpress.com/
The airport was purpose designed to be a very easy one to navigate through.
Luton airport is on the Thameslink line from the South coast calling at London Bridge, Blackfriars, City Thameslink, Farringdon and St Pancras (a 33 to 45 minute journey from St P), very frequent. That goes to Luton Airport Parkway
The train from London to Luton Airport Parkway [station code LTN] is £17.50 turn up and go but off peak advance fares can be found as low as £2.70.
From tomorrow the new DART fast automatic train opens (replacing a bus from the end of March) from the station for the 1 1/2 miles to the terminal, which you can buy inclusive of your ticket. It will take over from Heathrow Express as Britain's most expensive train at £4.90 for the ride.
To include the DART book to Luton Airport [station code LUA] and the DART is only a £2.40 add on for now (may change) although you can't currently add that on to a cheap advance fare, only to a full fare ticket (or that's what a test booking showed just now, press reports say it should be able to be added on to a cheap advance fare).
The new terminal is not long open. It is reputed to be a vast improvement on it's predecessor, which was awful (I've not been there, supposed to be there sometime in April, flying in from Inverness on a £16 Easyjet fare) although I have heard that it is a shopping mall with an airport added on, like so many modern airports. How fair that is I don't know.
On Ryanair the cheapest fare is an underseat bag only, level 2 fare is a cabin bag and an under seat bag [both to strict sizes] and level 3 fare is an under seat bag and checked bag.
