I agree there are loads of FPV monitors on the market nowadays so lots of choice, we have actually added a new 4 inch FPV montior today to our shop. At the end of the day the most important things to consider are the screen brightness as that will effect how well you can see the screen outdoors/ in direct sunlight.
Other minor issues to consider is the brand-name and performance. We find that flysight monitors outperform others in terms of their signal reliability. Our black pearl monitors also have the new longer range firmware which ensures they are more sensitive at longer ranges to pick up your video signal.
Here is a comparison between the Flysight Black Pearl and the Lilliput 329DW
The last thing to consider is the shop, do they offer decent support, warranty and returns if something does go wrong?
If you want to learn more about FPV we have a FPV gear guide which explains some of the basics, otherwise if you have any questions in particular just let me know.