From the pictures shown the GoolRC is a bottom end FlySky set. Both it and the Etrogo are very basic with only servo reversing switches and have no programming or memory for several models, you either need a new transmitter for each model or have to do a lot of set up to make all of your models identical as far as servos in the boat. The FlySky 6 is a more modern computer set which is more flexible and has several memories so each boat's setup can be stored and only one transmitter is needed.Definitely go for the FlySky, receivers are easily obtained when you build another boat. It is also using the AFDHS 2A protocol which is the latest version used by FlySky, the GoolRC is using the earlier protocol so receivers may be harder to find.Don't just go by cost, it is often better to spend a bit more at the start than find the cheapest option doesn't do all you need and have to then spend more to buy the better one later.
Jim