Assuming these are your horses, and not someone else's who is going to get extremely mad if their horses are critiqued in public.
The best way to choose a horse to buy, is to work with an instructor or trainer who knows how you ride, and would be giving you lessons once you buy the horse. A person like that has a reason to help you get a good horse.
When a person buys their own horse, unless they buy and sell horses every day, they usually run into problems and don't get the most suitable horse. A person who buys and sells a lot (like a trainer or instructor) has a big advantage.
There has been a lot written about evaluating conformation. But how to evaluate a horse really, 'it depends'. It depends on the intended use for the horse plus what the rider is like.
The horse for occasional pleasure riding can have some not-so-great features, where the horse that is going to be used a lot more often and harder, needs to be looked at in a very different way.
I would not worry a bit about getting horse #1. With some good food, worming, shoeing, teeth fixed by the horse dentist, and light exercise, that horse will look very different in a few months. She's real thin now and that makes a horse look bad. She not only lacks fat, she also lacks muscle. She looks like she hasn't had much exercise or food.
Then I would just have to see how she looked and acted when she felt better fed and in better shape. At that point I would decide what was the best 'job' for her. How a horse acts when it's out of shape can be a fooler. Many thin horses are very quiet and well mannered, and change when they feel better. So I would wait to decide what her 'job' would be, or who she'd be good for.
That horse, is not a 'grulla'. She's what you call dark brown, which is a really good color, because it always looks good, in the winter or the summer coat.
I would not worry about getting horse #2 either. But that horse looks pretty energetic and alert. Horses of her breeding usually are very energetic and even sometimes, sort of nervous and sensitive. I wouldn't get her for a beginner whose idea of a 'great ride' was a very, very slow walk once a month. That horse, at least in that photo, seems to have what's called a 'ewe neck'.
Normally I'd like a number of pictures from different angles, or a video, to be more sure. It might just be a bad picture of her. Or it might really be a ewe neck. Meaning there is a lot of thick muscle along the underside of her neck and not much of a curve to the topline of the neck, more of a downward curve. It's harder to ride a horse with that type of neck. It would take some skilled riding and a lot of patience to 'turn over' that sort of neck and get it to develop better. That's more a project for someone who has done a lot of that before.
Both horses are what is politely called 'a little easy in the topline', meaning their backs are not so strong looking. However, I would not worry about it until I saw both of them more trained and in better condition. I would not put a big heavy person on either horse, but kids are probably fine. It's important to get a saddle that fits that shape of back, and to put the saddle on the horse in the right way.
A lot of horses with not so nicely shaped backs, are used for years and years and years, and never give a moment's trouble. It is not a horse I would pick to go to the Olympics on, or do something real hard with, or put Hagred on, but I've seen horses shaped like that be school horses for many, many decades.