Both units are lovely, with full amenities, nice yard, and should rent very easily.

One of my tenants is going to do the yard work/maintenance regularly.

We have had some animal damage (large holes in the yard, damaged fences (just replaced a fence this past weekend that had used its life but it also had animal damage that I did not charge because the cross boards had significant weather rot so the animal made the fence look less attractive but likely did not shorten the life of the fence), some damage to flooring, damage to doors), but it has always been less than what we have collected due to the animals.A pet peeve (pun coincidental) is people with no medical issues claiming service animal. 

Of course assuming planning/zoning doesn't get in the way.The new trend here is row houses on tiny lots with no yard.

I am currently living with my GF at her place because she has a fenced in yard and together we have four dogs.

You can get a huge house, yard, garage, prime location, whatever you want practically, for $600k in Longmont, Broomfield, Lyons.

He’s going to be asking 12 dollars a sq ft lower than market avg in our area but, there is hardly no storage for seasonal decorations, (Christmas tree, fall decorations, etc) no garage, no fence for yard, and no place to store our dogs.

For example, more bedrooms, bigger back yard, closer to work or school. 

Is there a yard?  

If you don't have rental comps in the area, maybe get creative with those properties that are similar, and then assess how much the differences are worth (e.g. garage vs none, fenced yard, etc.).If you would really like to get an accurate assessment of what your rental's market price is, start a little above what you think it should be and see what kind of response you get from the online ads.