As a general rule the younger the rider (in terms of experience) the older the horse (in terms of experience)
A late gelded stallion is not a problem if the horse is well trained and well mannered. If it's a "nut job" (and any
breed can have them
) then age is irrelevant.
I'd be more interested in the experience of the older horse. If he was used as a gigalo
and the only time he came out of his stall was to get bred I'd look for something else. This means he's green and carries baggage. A novice rider does not
need a green horse with baggage.
On the other hand it he was a performance horse and regularly and frequently got worked in challenging environments (shows, parades, competitions, etc.) then I'd have very little worry about experience.
Temperment is another matter. If he was a quiet stallion then he'll be a quiet gelding. If he was an unrully stallion (and every
breed has them) then he'll be an unrully gelding.
Go and ride the two horses and then make your choice. If the older horse is, in fact, green then look and see if there's a Door # 3.
Good luck in your project.