How about for the dressage crowd?? Lol :)
I like the mare considerably more than the gelding although she will require a lot more retraining. In the conformation pictures the gelding is upright in the pasterns and has a straight hindleg, where as the mare has better hindlimb conformation, IMO. Chest width has been proven to have nothing to do with lung capacity, plus with more conditioning and filling out it may widen with muscle. I also like the overall balance of her better than the gelding, she appears to be well balanced over all four corners.
In the riding videos their differences in personality and life experiences become apparent. The mare is quite a bit more tense and high strung while the gelding is content to plunk around on the forehand complying with whatever his bouncy rider wants him to do. The gelding would be suitable for an AA looking to by something to learn to train, where as the mare would be more of a project. However I think her movement, while tense, is better than the gelding if only that she is more active in the hocks and is not dumping onto the forehand. Plus she looks very keen to be jumping which is a plus in an eventer.
Pending a clean vet check, and if you think you are experienced enough to be taking her on I think she would be the one of the two to progress higher and be more successful as an eventer. I would also get some body work done on her as the track does put a number on their bodies. Race horses are generally the one kind of horse I advocate for a good chiropractor to look at.
ETA - This is totally just my opinion though!! They are two different personality types and it is important to pick the one which you think will be most suitable for you and having your trainer present will help you pick the more suitable horse for you. IMO just looking for the better eventer, I like the mare, but the gelding may be more compatible for you.