They both work - IMO & an ex Avionics Service Manager it's the warranty & after warranty repairs that are of more interest to me & whether a manufacturer discontinues supporting a product.
Obviously there is a size & feature difference between them however is the big 'PRO' here is the warranty https://www.avidyne.com/product-warranty-registration/#tab-id-2
Dynon & Avidyne are excellent IMO - that's why I use them.
Try searching the web - you will find good info, if you haven't already.
I have a IFD550 on my HDX. Fantastic. I would strongly suggest you try their iPad simulator. I've got a lot of friends who are airline guys and they all say it is extremely similar to the heavy iron FMS.
Go with 540, much simpler to work than Garmin. And when you use the dynon, that instrument will be afterthough, used only during instrument approach, so will be easily remembered how to use. I hate garmin!