You can convert your 50k AA miles into 25k UA miles through Diners Club ($95 for one year, and you'll get bonus signup points worth 10k UA miles or so if you ONLY stay one year).

But if you think that's a good conversion because you'll get more likelihood of an award when you want it with 25k miles on UA, keep in mind that AA has "AAnyTime" awards for double the standard "PlanAAhead" awards, and so without converting at all you can get awards much easier if you're willing to use double the miles. (On the other hand, if you can use them for upgrades on paid flights you'd make on UA anyway, then yes, even at 1/2 the original value they may be worth more to you at UA than at AA.)

Having said all that, you mentioned that you found an award but that you could pay just a few hundred outright for the ticket. Well, isn't that often the case with US domestic tickets (especially if you're within reach of a Southwest-served airport at both ends)?

The real value of 25000 miles at UA it may net you a free flight worth $300ish, but that it can for example take a discount fare to Europe and upgrade one way of it to business class, which would cost you $$$$ if you tried to pay for it outright! There's not all that much value in free domestic flights if you use them for routes where there's a lot of low-cost competition, and yet you have all the award booking restrictions of low-cost flights. (The only domestic awards I'm really impressed with are walkup-equivalent, which can save you at least $600ish if truly used for last-minute opportunities for which you couldn't possibly have gotten any discount fare. But that's certainly not what 25k awards with ANY airline other than Southwest are.)