When did she say this, kind of wish she would have included that in her original post.
She didn't specifically say it, but I was working off the assumption that because she didn't say she was considering purchasing either of these mares, that she already owned them. Generally speaking, when people ask for a critique comparing horses they are considering purchasing, they include the price of the horse as well, because oft times that is a determining factor (does the horse's conformation make it worth the price the sellers are asking?). Also, some of the pics are taken in the same place.
Lol...my brain tends to notice odd things like similarities in location/background when looking at a video or picture. First time I watched Twilight, I leaned over to my friend and (never having been to the Pacific Northwest) said to my friend "This wasn't filmed in Washington, was it?" Yeah, noticed that the plates on Edward's Volvo were Oregon, no Washington.
Not knowing how your 4-H program is set up there, my advice may or may not be an option in your area. Most 4-H programs allow you to exhibit more than one animal. They generally have their halter classes split into age groups, here your 18 yo would be in the Aged Mares 16 & over and the younger would fall into the 5-9 yo mares. If it is set up similarly where you are, I'd say take them both. It would be more experience gained for both yourself & your mares.
If they just have a mare halter, no age, I'd go with the younger mare as she will have more chance to be competitive than the mare with more age would against younger stock. Get them both legged up & fit, hill work if you have access to hills is a great way to get them fit & build muscle & lots of trotting to tuck up your buckskin mare's tummy ;)