Felt and defy is slightly different style of riding.
defy is endurance bike and felt AR5 is aero bike so probably more like racing oriented bike.
when I see cassette felt has 12-25 and 18 defy has 11x32 and 19 defy has 11-34.
if you ride hills, 12x25 cassette will not be easy ( I got new road bike a few months ago and the bike is racing oriented bike with 11x25. My neighborhood has many steep hills and it was hard so I changed cassette to 11x30).

Defy vs AR5
if you need long range riding, Defy might be more comfortable since it’s Endurance bike.
if you need better brake performance, Defy’s disk brake will benefit.
Also felt components might be 5800 105 components vs new 7000 105 components if it matters to you.
Looks like defy might fit better for you but if you are felt fan AR5 is not bad choice.
personally, I might go with AR5 since I like aero bike but probably change some components such as cassette etc....

18 defy vs 19 defy
I don’t really compare between two but looks like cassette is only different.
cassette is not expensive parts and easy to replace so if you can get 18 defy for cheaper price, 18 defy will be better unless you have specific color preference haha.
black Friday is coming so you might find many close out deals. Good luck.

I was thinking of changing the cassette to a 11x30 but I was also going to wait till the spring to do that. I was thinking I could get used to it through indoor training (planning on doing 300+ miles per month till spring. My riding route are not too hilly but there are some unexplored areas north or me. The felt is less than 1700 so it sounded like a pretty decent deal.

The 18 defy is around 1500 and the 19 deft is 2000! Most of my riding would be on county roads as the city riding is terrible in my area.

the 18 defy is around 1500-1600 but it’s a travel to pick it up. The 19 defy is 2000 and the Felt is 1700. So the prices are close in range I guess it’s about feel.

The drawback if the felt is that it’s the 2016 model but other than that it looks and feels good just not a fan of the red trim. The 19 giant is slick with the grey and black color way.

You might want to go with the bike that allows for the widest tyre that can be fitted.

If you need unpaved road capability, 19 defy is the one since the other poster said new 19 defy can handle 32mm tire.
if you only ride good paved road, you need to chose between 18 defy vs 16 felt AR5.
18 defy vs 16 felt AR5
I guess, felt might be lighter than 18 defy (not sure about weight of both bikes.)
if you want better brake performance, 18 defy might be the one.
price difference is $200-300 so if you go with 18 defy, you will have extra money to upgrade wheelset.
Since defy is endurance bike, it should be more comfortable than felt especially longer distant riding.
negotiate with felt store to match 18 defy price. If you can do, I might go with felt AR5 (also I don’t often do long distant ride and prefer speed so that’s why I might go with felt). Felt AR5 (racing oriented) and defy (endurance) is different so you can make the decision from these aspects.
this video from GCN might help your decision.

I think 18 defy has 7000 105 components but felt might have 5800 105 components.

probably you might be right. 18 defy might still have old components.

From the photos on Giant's website, the 2019's brifters & derailleurs look like R7000 and the 2018's look like 5800. They both appear to have the same older style crankset.